fix clear notifications crash (#458)

app/src/main/java/com/keylesspalace/tusky/MainActivity.java

@@ -156,6 +156,10 @@ public class MainActivity extends BaseActivity implements ActionButtonActivity {
                 viewPager.setCurrentItem(tab.getPosition());
 
                 tintTab(tab, true);
+
+                if(tab.getPosition() == 1) {
+                    NotificationManager.clearNotifications(MainActivity.this);
+                }
             }
 
             @Override
@@ -210,6 +214,10 @@ public class MainActivity extends BaseActivity implements ActionButtonActivity {
                     .putBoolean("refreshProfileHeader", false)
                     .apply();
         }
+
+        if(viewPager.getCurrentItem() == 1) {
+            NotificationManager.clearNotifications(this);
+        }
     }
 
     @Override

app/src/main/java/com/keylesspalace/tusky/fragment/NotificationsFragment.java

@@ -250,16 +250,6 @@ public class NotificationsFragment extends SFragment implements
         super.onDestroyView();
     }
 
-    @Override
-    public void setUserVisibleHint(boolean isVisibleToUser) {
-        super.setUserVisibleHint(isVisibleToUser);
-        if (isVisibleToUser) {
-            //noinspection ConstantConditions
-            ((NotificationManager) getContext().getSystemService(Context.NOTIFICATION_SERVICE))
-                    .cancel(NotificationPullJobCreator.NOTIFY_ID);
-        }
-    }
-
     @Override
     public void onRefresh() {
         sendFetchNotificationsRequest(null, topId, FetchEnd.TOP, -1);

app/src/main/java/com/keylesspalace/tusky/util/NotificationManager.java

@@ -210,15 +210,17 @@ public class NotificationManager {
         }
     }
 
-    public static void clearNotifications(Context context) {
-        SharedPreferences notificationPreferences =
-                context.getSharedPreferences("Notifications", Context.MODE_PRIVATE);
-        notificationPreferences.edit().putString("current", "[]").apply();
-
-        android.app.NotificationManager manager = (android.app.NotificationManager)
-                context.getSystemService(Context.NOTIFICATION_SERVICE);
-        //noinspection ConstantConditions
-        manager.cancel(NotificationPullJobCreator.NOTIFY_ID);
+    public static void clearNotifications(@Nullable Context context) {
+        if(context != null) {
+            SharedPreferences notificationPreferences =
+                    context.getSharedPreferences("Notifications", Context.MODE_PRIVATE);
+            notificationPreferences.edit().putString("current", "[]").apply();
+
+            android.app.NotificationManager manager = (android.app.NotificationManager)
+                    context.getSystemService(Context.NOTIFICATION_SERVICE);
+            //noinspection ConstantConditions
+            manager.cancel(NotificationPullJobCreator.NOTIFY_ID);
+        }
     }
 
     private static boolean filterNotification(SharedPreferences preferences,