Fix NPE when opening Mapillary filter dialog

Author: Soumya1-byte

src/main/java/org/openstreetmap/josm/plugins/mapillary/gui/dialog/MapillaryFilterDialog.java

@@ -76,9 +76,18 @@
  *
  * @author nokutu
  */
+
+
 public final class MapillaryFilterDialog extends ToggleDialog
     implements OrganizationRecordListener, UserProfile.UserProfileListener, MVTTile.TileListener {
 
+    @Override
+    public void showDialog() {
+        if (!isShowing()) {
+            super.showDialog();
+        }
+    }
+
     @Serial
     private static final long serialVersionUID = -4192029663670922103L;