[skip ci] improve possibility to create a QApplication in a thread

src/App/Application.cpp

@@ -1889,7 +1889,7 @@ void Application::initConfig(int argc, char ** argv)
     Branding brand;
     QString binDir = QString::fromUtf8((mConfig["AppHomePath"] + "bin").c_str());
     QFileInfo fi(binDir, QString::fromLatin1("branding.xml"));
-    if (brand.readFile(fi.absoluteFilePath())) {
+    if (fi.exists() && brand.readFile(fi.absoluteFilePath())) {
         Branding::XmlConfig cfg = brand.getUserDefines();
         for (Branding::XmlConfig::iterator it = cfg.begin(); it != cfg.end(); ++it) {
             App::Application::Config()[it.key()] = it.value();

src/Base/Console.cpp

@@ -126,8 +126,6 @@ ConsoleSingleton::ConsoleSingleton(void)
   ,_defaultLogLevel(FC_LOGLEVEL_MSG)
 #endif
 {
-    // make sure this object is part of the main thread
-    ConsoleOutput::getInstance();
 }
 
 ConsoleSingleton::~ConsoleSingleton()
@@ -233,6 +231,11 @@ bool ConsoleSingleton::IsMsgTypeEnabled(const char* sObs, FreeCAD_ConsoleMsgType
 void ConsoleSingleton::SetConnectionMode(ConnectionMode mode)
 {
     connectionMode = mode;
+
+    // make sure this method gets called from the main thread
+    if (connectionMode == Queued) {
+        ConsoleOutput::getInstance();
+    }
 }
 
 /** Prints a Message

src/Main/FreeCADGuiPy.cpp

@@ -109,6 +109,9 @@ FreeCADGui_showMainWindow(PyObject * /*self*/, PyObject *args)
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 9, 0))
                 QApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
 #endif
+                // This only works well if the QApplication is the very first created instance
+                // of a QObject. Otherwise the application lives in a different thread than the
+                // main thread which will cause hazardous behaviour.
                 QtApplication app(argc, argv);
                 if (setupMainWindow()) {
                     app.exec();