Gui: fix possible crash when running the garbage collector after creating a shiboken wrapper

src/Gui/PythonWrapper.cpp

@@ -221,6 +221,8 @@ void registerTypes()
 // --------------------------------------------------------
 
 namespace Gui {
+template<typename qttype>
+PyTypeObject *getPyTypeObjectForTypeName();
 
 /*!
  * \brief The WrapperManager class
@@ -292,11 +294,29 @@ private:
         Base::PyGILStateLocker lock;
         wrappers.clear();
     }
+    void wrapQApplication()
+    {
+        // We have to explicitly hold a reference to the wrapper of the QApplication
+        // as otherwise it can happen that when running the gc the program crashes
+        // The code snippet below caused a crash on older versions:
+        // mw = Gui.getMainWindow()
+        // mw.style()
+        // import gc
+        // gc.collect()
+        PyTypeObject * type = getPyTypeObjectForTypeName<QApplication>();
+        if (type) {
+            auto sbk_type = reinterpret_cast<SbkObjectType*>(type);
+            std::string typeName = "QApplication";
+            PyObject* pyobj = Shiboken::Object::newObject(sbk_type, qApp, false, false, typeName.c_str());
+            addQObject(qApp, pyobj);
+        }
+    }
 
     WrapperManager()
     {
         connect(QApplication::instance(), &QCoreApplication::aboutToQuit,
                 this, &WrapperManager::clear);
+        wrapQApplication();
     }
     ~WrapperManager() = default;
 };