Fix GTK error wxApp::CleanUp() when GUI is not initialised

For the programs that use wxWidgets but can also run using just CLI, exiting
the program would show a bogus GTK error

GLib-GObject-CRITICAL **: g_type_class_unref: assertion 'g_class != NULL' failed

Fix this by only releasing the references to GTK_TYPE_WIDGET if we had
acquired it in the first place.

See https://github.com/wxWidgets/wxWidgets/pull/129

(this is a backport of e1f31febadea7d6277c7ee92c424ae7581e1d6d1 from master)

src/gtk/app.cpp

@@ -491,7 +491,9 @@ void wxApp::CleanUp()
         g_source_remove(m_idleSourceId);
 
     // release reference acquired by Initialize()
-    g_type_class_unref(g_type_class_peek(GTK_TYPE_WIDGET));
+    gpointer gt = g_type_class_peek(GTK_TYPE_WIDGET);
+    if (gt != NULL)
+        g_type_class_unref(gt);
 
     gdk_threads_leave();