Clear EhTagDatabase if it's not available

app/src/main/java/com/hippo/ehviewer/client/EhTagDatabase.java

@@ -145,8 +145,13 @@ public class EhTagDatabase {
   private static Lock lock = new ReentrantLock();
 
   @Nullable
-  public static EhTagDatabase getInstance() {
-    return instance;
+  public static EhTagDatabase getInstance(Context context) {
+    if (isPossible(context)) {
+      return instance;
+    } else {
+      instance = null;
+      return null;
+    }
   }
 
   @Nullable

app/src/main/java/com/hippo/ehviewer/ui/scene/GalleryDetailScene.java

@@ -869,7 +869,7 @@ public class GalleryDetailScene extends BaseScene implements View.OnClickListene
             mNoTags.setVisibility(View.GONE);
         }
 
-        EhTagDatabase ehTags = Settings.getShowTagTranslations() ? EhTagDatabase.getInstance() : null;
+        EhTagDatabase ehTags = Settings.getShowTagTranslations() ? EhTagDatabase.getInstance(context) : null;
         int colorTag = AttrResources.getAttrColor(context, R.attr.tagBackgroundColor);
         int colorName = AttrResources.getAttrColor(context, R.attr.tagGroupBackgroundColor);
         for (GalleryTagGroup tg : tagGroups) {