Null Object Reference when destroying something with an FlxAnimation on finishCallback

[Starveldt]

• Haxe version: 4.3.1
• Flixel version: 5.3.0
• OpenFL version: 9.2.1
• Lime version: 8.0.1
• Affected targets: Windows

---

I noticed this after upgrading every library & Haxe. Whenever an FlxSprite, or something with an FlxAnimationController gets destroyed (and even the FlxAnimationController itself getting destroyed) in a finishCallback function, it seemingly still updates after getting destroyed, and so throws a Null Object Reference error when trying to reference parent (now null). What confused me is how this used to work, but now it seemingly doesn't. I've got no further clue as to what could be causing the issue, other than just adding a simple null check to FlxAnimation's getCurrentFrameDuration() function. i.e:

function getCurrentFrameDuration()
{
	// The only change is "parent != null"
	final curframeDuration:Float = parent != null ? parent.getFrameDuration(frames[curFrame]) : 0;
	return curframeDuration > 0 ? curframeDuration : frameDuration;
}

---

Code snippet reproducing the issue:

package;

import flixel.FlxSprite;
import flixel.FlxState;

class PlayState extends FlxState
{
	override public function create():Void
	{
		var someSprite:FlxSprite = new FlxSprite();
		// insert animation code here
		someSprite.animation.finishCallback = function(anim:String) {
			trace('Finished animation');

			someSprite.kill();
			remove(someSprite, true);

			someSprite.destroy();
			someSprite = null;
		}
		
		add(someSprite);
		someSprite.animation.play('someAnimation', true);
	}
}

---

Error log:

Called from hxcpp::__hxcpp_main
Called from ApplicationMain::main ApplicationMain.hx line 26
Called from ApplicationMain::create ApplicationMain.hx line 135
Called from lime.app.Application::exec lime/app/Application.hx line 150
Called from lime._internal.backend.native.NativeApplication::exec lime/_internal/backend/native/NativeApplication.hx line 155
Called from lime._internal.backend.native.NativeApplication::handleRenderEvent lime/_internal/backend/native/NativeApplication.hx line 388
Called from lime.app._Event_lime_graphics_RenderContext_Void::dispatch lime/_internal/macros/EventMacro.hx line 91
Called from openfl.display.Stage::__onLimeRender openfl/display/Stage.hx line 2046
Called from openfl.display.Stage::__broadcastEvent openfl/display/Stage.hx line 1181
Called from openfl.display.DisplayObject::__dispatch openfl/display/DisplayObject.hx line 1399
Called from openfl.events.EventDispatcher::__dispatchEvent openfl/events/EventDispatcher.hx line 402
Called from flixel.FlxGame::onEnterFrame flixel/FlxGame.hx line 556
Called from flixel.FlxGame::step flixel/FlxGame.hx line 682
Called from flixel.FlxGame::update flixel/FlxGame.hx line 752
Called from flixel.FlxState::tryUpdate flixel/FlxState.hx line 220
Called from states.PlayState::update states/PlayState.hx line 2143
Called from states.MusicBeatState::update states/MusicBeatState.hx line 72
Called from flixel.addons.ui.FlxUIState::update flixel/addons/ui/FlxUIState.hx line 212
Called from flixel.group.FlxTypedGroup::update flixel/group/FlxGroup.hx line 170
Called from flixel.group.FlxTypedSpriteGroup::update flixel/group/FlxSpriteGroup.hx line 205
Called from flixel.group.FlxTypedGroup::update flixel/group/FlxGroup.hx line 170
Called from flixel.FlxSprite::update flixel/FlxSprite.hx line 758
Called from flixel.FlxSprite::updateAnimation flixel/FlxSprite.hx line 766
Called from flixel.animation.FlxAnimationController::update flixel/animation/FlxAnimationController.hx line 93
Called from flixel.animation.FlxAnimation::update flixel/animation/FlxAnimation.hx line 220
Called from flixel.animation.FlxAnimation::getCurrentFrameDuration flixel/animation/FlxAnimation.hx line 227
Critical Error: Uncatchable Throw: Null Object Reference

(small note: NativeApplication.hx line 388 is actually window.onRender.dispatch(window.context); - i modified NativeApplication to have better fullscreening but that's it)

Unless this was intentional, I've got no idea how to fix the issue beyond adding a null check to getCurrentFrameDuration() (and most likely other functions...)

Any help is appreciated.

[Geokureli]

You shouldnt really ever destroy a sprite, call kill() instead and let it be destroyed automatically when that state is destroyed

[Starveldt]

You shouldnt really ever destroy a sprite, call kill() instead and let it be destroyed automatically when that state is destroyed

I wasn't aware that you aren't usually supposed to destroy sprites... I usually only do it to free up memory since it can get quite memory intensive, especially in certain states that spawn & remove hundreds of sprites in realtime. Are there other options, or do I just have to bite the bullet?

[Geokureli]

Are there other options, or do I just have to bite the bullet?

Are you sure that calling destroy is actually clearing up any memory, or are you just assuming this? FlxSprites don't take up a lot of memory, and destroying them doesn't unload the corresponding graphics. if you have 100's of new FlxSprites being created and killed every few seconds (like bullets in a shoot'em up) than consider object pooling via FlxGroup's recycle function, but I'm skeptical that calling destroy is really helping you

[Geokureli]

FYI I still plan to look into this null object reference error, I just don't recommend what you're doing. a quick fix is to call kill instead, or remove the sprite before destroying it

[Starveldt]

FlxSprites don't take up a lot of memory, and destroying them doesn't unload the corresponding graphics.

I see, I thought it did do so, but I guess not. Also, I do use the recycle function, though I did think that destroy() helped a little bit, since I was reading memory usage being slightly lower when I did(?). I'm more used to having to destroy things in other game engines aswell which is what made me think so. I didn't know you were just supposed to only call kill().

a quick fix is to call kill instead, or remove the sprite before destroying it

I actually do call kill() and remove(sprite, true) before I destroy the sprite, which is why I was dumbfounded when this error occured after updating Flixel to 5.3.0, since it did used to work. i.e:

someSprite.kill();
remove(someSprite, true);
// destroy it now that it's completely out of the group
someSprite.destroy();

Though I do now just realize that I forgot to include kill() and remove() in the example I put in the OP.

[Geokureli]

You should definitely never call destroy on sprites you plan to recycle, btw