Add possibility to check if OsEng is instantiated

modules/onscreengui/onscreenguimodule.cpp

@@ -49,6 +49,15 @@ OnScreenGUIModule::OnScreenGUIModule()
 {
     addPropertySubOwner(gui);
 
+    // TODO: Remove dependency on OsEng.
+    // Instead, make this class implement an interface that OsEng depends on.
+    // Do not try to register module callbacks if OsEng does not exist,
+    // for example in the TaskRunner.
+
+    if (!OpenSpaceEngine::isCreated()) {
+        return;
+    }
+
     OsEng.registerModuleCallback(
         OpenSpaceEngine::CallbackOption::Initialize,
         [](){