frontend: Abort if obs-transitions failed to load

The frontend UI assumes transitions are always available and crashes if
none are present. Resolving the crashes resulted in a completely broken
UI, so it's safe to assume that transitions are a core requirement for a
working OBS.

Crash reports indicate that obs-transitions is failing to load for some
users, so we should handle the case where no transitions load and abort
with an error instead of crashing.

frontend/widgets/OBSBasic.hpp

@@ -1500,8 +1500,8 @@ private:
 	std::unordered_map<std::string, std::string> transitionNameToUuids;
 	int transitionDuration;
 	std::string currentTransitionUuid;
-	obs_source_t *fadeTransition;
-	obs_source_t *cutTransition;
+	obs_source_t *fadeTransition = nullptr;
+	obs_source_t *cutTransition = nullptr;
 	std::vector<QuickTransition> quickTransitions;
 	bool swapScenesMode = true;
 

frontend/widgets/OBSBasic_Transitions.cpp

@@ -73,6 +73,14 @@ void OBSBasic::InitDefaultTransitions()
 		}
 	}
 
+	// We require transitions in order to function, so exit with an error if
+	// obs-transitions failed to load for whatever reason.
+	if (!fadeTransition || !cutTransition) {
+		// FIXME: https://github.com/obsproject/obs-studio/issues/13394
+		throw "InitDefaultTransitions: Could not load default transitions. Try re-installing OBS Studio from "
+		      "<a href=\"https://obsproject.com/\">obsproject.com</a>.";
+	}
+
 	for (OBSSource &tr : defaultTransitions) {
 		std::string uuid = obs_source_get_uuid(tr);