UI: Add null checks before doing some API calls

Harmless, but generated a debug warning for null pointers passed into
the API.

UI/window-basic-main-transitions.cpp

@@ -445,7 +445,8 @@ void OBSBasic::SetTransition(OBSSource transition)
 	ui->transitionDurationLabel->setVisible(!fixed);
 	ui->transitionDuration->setVisible(!fixed);
 
-	bool configurable = obs_source_configurable(transition);
+	bool configurable = transition ? obs_source_configurable(transition)
+				       : false;
 	ui->transitionRemove->setEnabled(configurable);
 	ui->transitionProps->setEnabled(configurable);
 

UI/window-basic-main.cpp

@@ -8639,7 +8639,7 @@ void OBSBasic::UpdateEditMenu()
 	const bool canTransformSingle = videoCount == 1 && totalCount == 1;
 
 	OBSSceneItem curItem = GetCurrentSceneItem();
-	bool locked = obs_sceneitem_locked(curItem);
+	bool locked = curItem && obs_sceneitem_locked(curItem);
 
 	ui->actionCopySource->setEnabled(totalCount > 0);
 	ui->actionEditTransform->setEnabled(canTransformSingle && !locked);

UI/window-basic-preview.cpp

@@ -654,7 +654,7 @@ void OBSBasicPreview::mousePressEvent(QMouseEvent *event)
 
 void OBSBasicPreview::UpdateCursor(uint32_t &flags)
 {
-	if (obs_sceneitem_locked(stretchItem)) {
+	if (!stretchItem || obs_sceneitem_locked(stretchItem)) {
 		unsetCursor();
 		return;
 	}