UI: Use qobject_cast when getting OBSBasic

Since we are converting between qobjects, it is safer to use
qobject_cast than reinterpret_cast.

UI/window-basic-main.cpp

@@ -9688,7 +9688,7 @@ void OBSBasic::RepairCustomExtraDockName()
 
 OBSBasic *OBSBasic::Get()
 {
-	return reinterpret_cast<OBSBasic *>(App()->GetMainWindow());
+	return qobject_cast<OBSBasic *>(App()->GetMainWindow());
 }
 
 bool OBSBasic::StreamingActive()