Prevent UIs from being shown twice (fixes ardour double window)

source/backend/engine/CarlaEngineNative.cpp

@@ -1181,6 +1181,9 @@ protected:
     {
         if (show)
         {
+            if (fUiServer.isOk())
+                return;
+
             CarlaString path(pHost->resourceDir);
             path += "/carla-plugin";
 

source/modules/CarlaNativeExtUI.hpp

@@ -51,6 +51,9 @@ protected:
     {
         if (show)
         {
+            if (isOk())
+                return;
+
             CarlaString path(getResourceDir() + fExtUiPath);
             carla_stdout("Trying to start UI using \"%s\"", path.buffer());
 

source/modules/native-plugins/zynaddsubfx-synth.cpp

@@ -447,8 +447,6 @@ protected:
                     Fl_Theme::set("Cairo");
                 }
 
-                CARLA_ASSERT(fUi == nullptr);
-
                 if (fUi == nullptr)
                 {
                     fUiClosed = 0;
@@ -456,6 +454,8 @@ protected:
                     fUi->masterwindow->label(kHost->uiName);
                     fUi->showUI();
                 }
+                else
+                    fUi->showUI();
             }
             else if (fNextUiAction == 0) // close
             {