re-fix resetShowingDesktop invocation

was fixed with f9e18206
broke it with 3433b31a

REVIEW: 108452
BUG: 302248

kwin/client.cpp

@@ -1096,7 +1096,8 @@ void Client::updateVisibility()
             internalHide(Allowed);
         return;
     }
-    resetShowingDesktop(true);
+    if (isManaged())
+        resetShowingDesktop(true);
     internalShow(Allowed);
 }