Update the gdscript logic used to retrieve the GDExtension singletons

demo/main.gd

@@ -1,7 +1,6 @@
 extends Node3D
 
 var xr_interface : XRInterface = null
-var scene_capture: OpenXRFbSceneCaptureExtensionWrapper = null
 
 # Called when the node enters the scene tree for the first time.
 func _ready():
@@ -10,15 +9,17 @@ func _ready():
 		var vp: Viewport = get_viewport()
 		vp.use_xr = true
 
-	scene_capture = Engine.get_singleton("OpenXRFbSceneCaptureExtensionWrapper")
-	scene_capture.connect("scene_capture_completed", _on_scene_capture_completed)
+	var scene_capture := Engine.get_singleton("OpenXRFbSceneCaptureExtensionWrapper")
+	if scene_capture:
+		scene_capture.connect("scene_capture_completed", _on_scene_capture_completed)
 
 
 func _on_scene_capture_completed():
 	print("Scene Capture completed")
 
 
 func _on_left_hand_button_pressed(name):
+	var scene_capture := Engine.get_singleton("OpenXRFbSceneCaptureExtensionWrapper")
 	if name == "menu_button" and scene_capture:
 		print("Triggering scene capture")
 		scene_capture.request_scene_capture()