Fix possible crash

app/src/main/java/com/hippo/ehviewer/preference/TaskPreference.java

@@ -72,8 +72,10 @@ public abstract class TaskPreference extends DialogPreference {
     protected void onTaskEnd() {
         // Dismiss dialog
         Dialog dialog = getDialog();
-        if (null != dialog) {
+        try {
             dialog.dismiss();
+        } catch (Exception e) {
+            e.printStackTrace();
         }
         // Clear async
         mTask = null;

app/src/main/java/com/hippo/ehviewer/ui/FilterActivity.java

@@ -143,6 +143,7 @@ public class FilterActivity extends ToolbarActivity {
                 .setTitle(R.string.add_filter)
                 .setView(R.layout.dialog_add_filter)
                 .setPositiveButton(R.string.add, null)
+                .setNegativeButton(android.R.string.cancel, null)
                 .show();
         AddFilterDialogHelper helper = new AddFilterDialogHelper();
         helper.setDialog(dialog);
@@ -161,7 +162,9 @@ public class FilterActivity extends ToolbarActivity {
                         mAdapter.notifyDataSetChanged();
                     }
                     updateView(true);
-                }).show();
+                })
+                .setNegativeButton(android.R.string.cancel, null)
+                .show();
     }
 
     private class AddFilterDialogHelper implements View.OnClickListener {

app/src/main/java/com/hippo/ehviewer/ui/HostsActivity.java

@@ -213,6 +213,7 @@ public class HostsActivity extends ToolbarActivity
         protected void onCreateDialogBuilder(AlertDialog.Builder builder) {
             builder.setTitle(R.string.add_host);
             builder.setPositiveButton(R.string.add_host_add, null);
+            builder.setNegativeButton(android.R.string.cancel, null);
         }
 
         @Override