rich workspace not in folder picker activity

Signed-off-by: tobiasKaminsky <tobias@kaminsky.me>

src/main/java/com/owncloud/android/ui/fragment/OCFileListFragment.java

@@ -539,7 +539,7 @@ public class OCFileListFragment extends ExtendedListFragment implements
 
     @Override
     public void onHeaderClicked() {
-        if (!getAdapter().isMultiSelect()) {
+        if (!getAdapter().isMultiSelect() && mContainerActivity instanceof FileDisplayActivity) {
             ((FileDisplayActivity) mContainerActivity).startRichWorkspacePreview(getCurrentFile());
         }
     }