Fix disabled gui when stopping engine with no plugins

Signed-off-by: falkTX <falktx@falktx.com>

source/frontend/carla_host.py

@@ -874,7 +874,7 @@ class HostWindow(QMainWindow):
         if self.host.is_engine_running():
             if self.fCustomStopAction == self.CUSTOM_ACTION_PROJECT_LOAD:
                 self.removeAllPlugins()
-            else:
+            elif self.fPluginCount != 0:
                 self.fCurrentlyRemovingAllPlugins = True
                 self.projectLoadingStarted()