Fix a build warning against Qt 5.15

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

source/bridges-ui/CarlaBridgeToolkitQt.cpp

@@ -91,7 +91,7 @@ public:
 
         fApp = new QApplication(qargc, qargv);
 
-        fWindow = new QMainWindow(nullptr, nullptr);
+        fWindow = new QMainWindow();
         fWindow->resize(30, 30);
         fWindow->hide();