diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/EhCallback.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/EhCallback.kt
index 0a92574cb..64073a669 100644
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/EhCallback.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/EhCallback.kt
@@ -13,57 +13,53 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+package com.hippo.ehviewer.ui.scene
 
-package com.hippo.ehviewer.ui.scene;
+import android.content.Context
+import android.widget.Toast
+import androidx.annotation.StringRes
+import androidx.fragment.app.Fragment
+import com.hippo.ehviewer.EhApplication
+import com.hippo.ehviewer.client.EhClient
+import com.hippo.ehviewer.ui.MainActivity
 
-import android.content.Context;
-import android.widget.Toast;
+abstract class EhCallback<E : Fragment?, T>(context: Context) : EhClient.Callback<T> {
+    val application: EhApplication
 
-import androidx.annotation.StringRes;
-import androidx.fragment.app.Fragment;
-
-import com.hippo.ehviewer.EhApplication;
-import com.hippo.ehviewer.client.EhClient;
-import com.hippo.ehviewer.ui.EhActivity;
-import com.hippo.ehviewer.ui.MainActivity;
-
-public abstract class EhCallback<E extends Fragment, T> implements EhClient.Callback<T> {
-
-    private final EhApplication mApplication;
-
-    public EhCallback(Context context) {
-        mApplication = (EhApplication) context.getApplicationContext();
+    init {
+        application = context.applicationContext as EhApplication
     }
 
-    public Context getContent() {
-        Context context = mApplication.getTopActivity();
-        if (context == null) {
-            context = getApplication();
+    val content: Context
+        get() {
+            var context: Context? = application.topActivity
+            if (context == null) {
+                context = application
+            }
+            return context
         }
-        return context;
-    }
 
-    public EhApplication getApplication() {
-        return mApplication;
-    }
-
-    public void showTip(@StringRes int id, int length) {
-        EhActivity activity = mApplication.getTopActivity();
-        if (activity instanceof MainActivity) {
-            ((MainActivity) activity).showTip(id, length);
+    fun showTip(@StringRes id: Int, length: Int) {
+        val activity = application.topActivity
+        if (activity is MainActivity) {
+            activity.showTip(id, length)
         } else {
-            Toast.makeText(getApplication(), id,
-                    length == BaseScene.LENGTH_LONG ? Toast.LENGTH_LONG : Toast.LENGTH_SHORT).show();
+            Toast.makeText(
+                application, id,
+                if (length == BaseScene.LENGTH_LONG) Toast.LENGTH_LONG else Toast.LENGTH_SHORT
+            ).show()
         }
     }
 
-    public void showTip(String tip, int length) {
-        EhActivity activity = mApplication.getTopActivity();
-        if (activity instanceof MainActivity) {
-            ((MainActivity) activity).showTip(tip, length);
+    fun showTip(tip: String, length: Int) {
+        val activity = application.topActivity
+        if (activity is MainActivity) {
+            activity.showTip(tip, length)
         } else {
-            Toast.makeText(getApplication(), tip,
-                    length == BaseScene.LENGTH_LONG ? Toast.LENGTH_LONG : Toast.LENGTH_SHORT).show();
+            Toast.makeText(
+                application, tip,
+                if (length == BaseScene.LENGTH_LONG) Toast.LENGTH_LONG else Toast.LENGTH_SHORT
+            ).show()
         }
     }
-}
+}
\ No newline at end of file
diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryCommentsScene.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryCommentsScene.kt
index b8e413757..0e01c0b6d 100644
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryCommentsScene.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryCommentsScene.kt
@@ -700,7 +700,7 @@ class GalleryCommentsScene : BaseToolbarScene(), View.OnClickListener, OnRefresh
     private class ProgressCommentHolder(inflater: LayoutInflater, parent: ViewGroup?) :
         CommentHolder(inflater, R.layout.item_gallery_comment_progress, parent)
 
-    private inner class RefreshCommentListener(context: Context?) :
+    private inner class RefreshCommentListener(context: Context) :
         EhCallback<GalleryCommentsScene?, GalleryDetail>(context) {
         override fun onSuccess(result: GalleryDetail) {
             val scene = this@GalleryCommentsScene
@@ -715,7 +715,7 @@ class GalleryCommentsScene : BaseToolbarScene(), View.OnClickListener, OnRefresh
         override fun onCancel() {}
     }
 
-    private inner class CommentGalleryListener(context: Context?, private val mCommentId: Long) :
+    private inner class CommentGalleryListener(context: Context, private val mCommentId: Long) :
         EhCallback<GalleryCommentsScene?, GalleryCommentList>(context) {
         override fun onSuccess(result: GalleryCommentList) {
             showTip(
@@ -738,7 +738,7 @@ class GalleryCommentsScene : BaseToolbarScene(), View.OnClickListener, OnRefresh
         override fun onCancel() {}
     }
 
-    private inner class VoteCommentListener(context: Context?) :
+    private inner class VoteCommentListener(context: Context) :
         EhCallback<GalleryCommentsScene?, VoteCommentParser.Result>(context) {
         override fun onSuccess(result: VoteCommentParser.Result) {
             showTip(
diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryDetailScene.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryDetailScene.kt
index 790651bfd..df5a2b98f 100755
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryDetailScene.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryDetailScene.kt
@@ -1259,7 +1259,7 @@ class GalleryDetailScene : CollapsingToolbarScene(), View.OnClickListener, Downl
     @IntDef(STATE_INIT, STATE_NORMAL, STATE_REFRESH, STATE_REFRESH_HEADER, STATE_FAILED)
     @Retention(AnnotationRetention.SOURCE)
     private annotation class State
-    private class VoteTagListener(context: Context?) :
+    private class VoteTagListener(context: Context) :
         EhCallback<GalleryDetailScene?, VoteTagParser.Result>(context) {
         override fun onSuccess(result: VoteTagParser.Result) {
             if (!TextUtils.isEmpty(result.error)) {
@@ -1277,7 +1277,7 @@ class GalleryDetailScene : CollapsingToolbarScene(), View.OnClickListener, Downl
     }
 
     private class DownloadArchiveListener(
-        context: Context?,
+        context: Context,
         private val mGalleryInfo: GalleryInfo?
     ) : EhCallback<GalleryDetailScene?, String>(context) {
         override fun onSuccess(result: String) {
@@ -1338,7 +1338,7 @@ class GalleryDetailScene : CollapsingToolbarScene(), View.OnClickListener, Downl
         }
     }
 
-    private inner class GetGalleryDetailListener(context: Context?) :
+    private inner class GetGalleryDetailListener(context: Context) :
         EhCallback<GalleryDetailScene?, GalleryDetail>(context) {
         override fun onSuccess(result: GalleryDetail) {
             application.removeGlobalStuff(this)
@@ -1366,7 +1366,7 @@ class GalleryDetailScene : CollapsingToolbarScene(), View.OnClickListener, Downl
     }
 
     private inner class RateGalleryListener(
-        context: Context?
+        context: Context
     ) : EhCallback<GalleryDetailScene?, RateGalleryParser.Result>(context) {
         override fun onSuccess(result: RateGalleryParser.Result) {
             showTip(R.string.rate_successfully, LENGTH_SHORT)
@@ -1383,7 +1383,7 @@ class GalleryDetailScene : CollapsingToolbarScene(), View.OnClickListener, Downl
     }
 
     private inner class ModifyFavoritesListener(
-        context: Context?,
+        context: Context,
         private val mAddOrRemove: Boolean
     ) :
         EhCallback<GalleryDetailScene?, Void?>(context) {
diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryPreviewsScene.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryPreviewsScene.kt
index 9da0aef71..d3ff9284c 100644
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryPreviewsScene.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryPreviewsScene.kt
@@ -208,7 +208,7 @@ class GalleryPreviewsScene : BaseToolbarScene() {
     }
 
     private inner class GetPreviewSetListener(
-        context: Context?,
+        context: Context,
         private val mTaskId: Int
     ) : EhCallback<GalleryPreviewsScene, Pair<PreviewSet, Int>>(context) {
         override fun onSuccess(result: Pair<PreviewSet, Int>) {
diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/HistoryScene.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/HistoryScene.kt
index cc78be596..5fe4fe6b3 100644
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/HistoryScene.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/HistoryScene.kt
@@ -318,7 +318,7 @@ class HistoryScene : BaseToolbarScene() {
         return true
     }
 
-    private class AddToFavoriteListener(context: Context?) :
+    private class AddToFavoriteListener(context: Context) :
         EhCallback<GalleryListScene?, Void?>(context) {
         override fun onSuccess(result: Void?) {
             showTip(R.string.add_to_favorite_success, LENGTH_SHORT)
@@ -331,7 +331,7 @@ class HistoryScene : BaseToolbarScene() {
         override fun onCancel() {}
     }
 
-    private class RemoveFromFavoriteListener(context: Context?) :
+    private class RemoveFromFavoriteListener(context: Context) :
         EhCallback<GalleryListScene?, Void?>(context) {
         override fun onSuccess(result: Void?) {
             showTip(R.string.remove_from_favorite_success, LENGTH_SHORT)
diff --git a/app/src/main/java/com/hippo/ehviewer/ui/scene/ProgressScene.kt b/app/src/main/java/com/hippo/ehviewer/ui/scene/ProgressScene.kt
index b33e76176..bd088eb6f 100644
--- a/app/src/main/java/com/hippo/ehviewer/ui/scene/ProgressScene.kt
+++ b/app/src/main/java/com/hippo/ehviewer/ui/scene/ProgressScene.kt
@@ -184,7 +184,7 @@ class ProgressScene : BaseScene(), View.OnClickListener {
     }
 
     private inner class GetGalleryTokenListener(
-        context: Context?
+        context: Context
     ) : EhCallback<ProgressScene, String>(context) {
         override fun onSuccess(result: String) {
             val scene = this@ProgressScene