fix nullability of getDarkThemeMode function

Signed-off-by: alperozturk <alper_ozturk@proton.me>

app/src/main/java/com/nextcloud/client/preferences/AppPreferencesImpl.java

@@ -437,7 +437,13 @@ public final class AppPreferencesImpl implements AppPreferences {
 
     @Override
     public boolean isDarkModeEnabled() {
-        return getDarkThemeMode() == DarkMode.DARK;
+        DarkMode darkMode = getDarkThemeMode();
+
+        if (darkMode != null) {
+            return getDarkThemeMode() == DarkMode.DARK;
+        } else {
+            return false;
+        }
     }
 
     @Override

app/src/main/java/com/owncloud/android/utils/DisplayUtils.java

@@ -856,7 +856,7 @@ public final class DisplayUtils {
             stopShimmer(shimmerThumbnail, thumbnailView);
 
             boolean isAutoUploadFolder = SyncedFolderProvider.isAutoUploadFolder(syncedFolderProvider, file, user);
-            boolean isDarkModeActive = preferences.getDarkThemeMode() == DarkMode.DARK;
+            boolean isDarkModeActive = preferences.isDarkModeEnabled();
 
             Integer overlayIconId = file.getFileOverlayIconId(isAutoUploadFolder);
             LayerDrawable fileIcon = MimeTypeUtil.getFileIcon(isDarkModeActive, overlayIconId, context, viewThemeUtils);