Fix segfault on quit (#6212)

soh/soh/OTRGlobals.cpp

@@ -1556,6 +1556,7 @@ extern "C" void DeinitOTR() {
     // Destroying gui here because we have shared ptrs to LUS objects which output to SPDLOG which is destroyed before
     // these shared ptrs.
     SohGui::Destroy();
+    sohFast3dWindow = nullptr;
 
     OTRGlobals::Instance->context = nullptr;
 }