Fix lint and spotbugs

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/src/main/AndroidManifest.xml

@@ -227,11 +227,6 @@
             android:name=".ui.preview.PreviewImageActivity"
             android:exported="false"
             android:theme="@style/Theme.ownCloud.Overlay" />
-        <activity
-            android:name=".ui.preview.PreviewVideoActivity"
-            android:exported="false"
-            android:theme="@style/Theme.ownCloud.Fullscreen" />
-
         <service
             android:name=".authentication.AccountAuthenticatorService"
             android:exported="false">

app/src/main/java/com/owncloud/android/ui/preview/PreviewMediaFragment.java

@@ -86,6 +86,7 @@ import androidx.annotation.NonNull;
 import androidx.annotation.StringRes;
 import androidx.appcompat.widget.AppCompatImageButton;
 import androidx.drawerlayout.widget.DrawerLayout;
+import androidx.fragment.app.FragmentActivity;
 import androidx.fragment.app.FragmentManager;
 
 /**
@@ -605,7 +606,10 @@ public class PreviewMediaFragment extends FileFragment implements OnTouchListene
     }
 
     private void startFullScreenVideo() {
-        new PreviewVideoFullscreenDialog(getActivity(), exoPlayer, binding.exoplayerView).show();
+        final FragmentActivity activity = getActivity();
+        if (activity != null) {
+            new PreviewVideoFullscreenDialog(activity, exoPlayer, binding.exoplayerView).show();
+        }
     }
 
     @Override