qt4: destroy the variable only if the variable does exist.

modules/gui/qt4/qt4.cpp

@@ -334,7 +334,9 @@ static void Close( vlc_object_t *p_this )
     intf_thread_t *p_intf = (intf_thread_t *)p_this;
     intf_sys_t *p_sys = p_intf->p_sys;
 
-    var_Destroy (p_this->p_libvlc, "qt4-iface");
+    if( !p_sys->b_isDialogProvider )
+        var_Destroy (p_this->p_libvlc, "qt4-iface");
+
     QVLCApp::triggerQuit();
 
     vlc_join (p_sys->thread, NULL);