fix warnings to trigger CI build

src/main/java/com/owncloud/android/ui/preview/PreviewImageFragment.java

@@ -141,7 +141,6 @@ public class PreviewImageFragment extends FileFragment {
         return frag;
     }
 
-
     /**
      * Creates an empty fragment for image previews.
      *
@@ -155,10 +154,6 @@ public class PreviewImageFragment extends FileFragment {
         mIgnoreFirstSavedState = false;
     }
 
-
-    /**
-     * {@inheritDoc}
-     */
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
@@ -172,10 +167,6 @@ public class PreviewImageFragment extends FileFragment {
         setHasOptionsMenu(true);
     }
 
-
-    /**
-     * {@inheritDoc}
-     */
     @Override
     public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
         super.onCreateView(inflater, container, savedInstanceState);
@@ -219,9 +210,6 @@ public class PreviewImageFragment extends FileFragment {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     */
     @Override
     public void onSaveInstanceState(@NonNull Bundle outState) {
         super.onSaveInstanceState(outState);
@@ -718,7 +706,9 @@ public class PreviewImageFragment extends FileFragment {
      */
     private void finish() {
         Activity container = getActivity();
-        container.finish();
+        if (container != null) {
+            container.finish();
+        }
     }
 
     private void togglePreviewImageFullScreen() {