Add possibility to check if OsEng is instantiated

include/openspace/engine/openspaceengine.h

@@ -78,6 +78,7 @@ public:
         std::vector<std::string>& sgctArguments, bool& requestClose);
     static void destroy();
     static OpenSpaceEngine& ref();
+    static bool isCreated();
 
     double runTime();
     void setRunTime(double t);

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,
         [](){

src/engine/openspaceengine.cpp

@@ -185,6 +185,10 @@ OpenSpaceEngine& OpenSpaceEngine::ref() {
     return *_engine;
 }
 
+bool OpenSpaceEngine::isCreated() {
+    return _engine != nullptr;
+}
+
 void OpenSpaceEngine::create(int argc, char** argv,
                              std::unique_ptr<WindowWrapper> windowWrapper,
                              std::vector<std::string>& sgctArguments, bool& requestClose)