Fix a crash when toggling back and forth between a generic plugin GUI window and a VST plugin's own GUI

gtk2_ardour/windows_vst_plugin_ui.cc

@@ -58,6 +58,8 @@ WindowsVSTPluginUI::WindowsVSTPluginUI (boost::shared_ptr<PluginInsert> pi, boos
 
 WindowsVSTPluginUI::~WindowsVSTPluginUI ()
 {
+	fst_destroy_editor (_vst->state());
+
 	// plugin destructor destroys the custom GUI, via Windows fun-and-games,
 	// and then our PluginUIWindow does the rest
 }

libs/fst/vstwin.c

@@ -374,6 +374,9 @@ fst_exit (void)
 int
 fst_run_editor (VSTState* fst, void* window_parent)
 {
+	/* For safety, remove any pre-existing editor window */ 
+	fst_destroy_editor (fst);
+	
 	if (fst->windows_window == NULL) {
 		HMODULE hInst;
 		HWND window;