[Client] Ensure mwmp::Main is initialized before GUIController cleanup

Previously, certain errors early in the program's execution – such as an attempt to use two different packet processors for the same packet ID – would not be displayed because of a crash when attempting to get an uninitialized mwmp::Main in the Engine's destructor.

apps/openmw/engine.cpp

@@ -345,8 +345,10 @@ OMW::Engine::~Engine()
     /*
         Start of tes3mp addition
 
-        Free up memory allocated by multiplayer's GUIController
+        Free up memory allocated by multiplayer's GUIController, but make sure
+        mwmp::Main has actually been initialized
     */
+    if (mwmp::Main::isInitialized())
         mwmp::Main::get().getGUIController()->cleanUp();
     /*
         End of tes3mp addition

apps/openmw/mwmp/Main.cpp

@@ -163,6 +163,11 @@ void Main::postInit()
     MWBase::Environment::get().getMechanicsManager()->toggleAI();
 }
 
+bool Main::isInitialized()
+{
+    return pMain != nullptr;
+}
+
 void Main::destroy()
 {
     assert(pMain);

apps/openmw/mwmp/Main.hpp

@@ -23,6 +23,7 @@ namespace mwmp
         static void configure(const boost::program_options::variables_map &variables);
         static bool init(std::vector<std::string> &content, Files::Collections &collections);
         static void postInit();
+        static bool isInitialized();
         static void destroy();
         static const Main &get();
         static void frame(float dt);