MapBrowserEventHandler needs to be destroyed for listeners to be unregistered

Describe the bug MouseWheelZoom.conditionInternal_ throws errors when the map target is null due to the hasAttribute check. I see this is changed on master but I believe this bug will still exist (https://github.com/openlayers/openlayers/commit/2cac6c4f39521470a899f1c6338aa47baea05392#diff-a08a90960184e04e1b7384a480a81d6a).

To Reproduce Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/react-openlayers-xni5f?file=/src/App.js which contains a simple map example reloading every 5s and setting the previous map target to null (myMap.setTarget(null);)
2. Click on the map to drag but don’t release for 5s.
3. Next mouse movements will trigger an error:

What I’ve found so far is

• Events need a map target or they crash
• Events are not deregisterd without map.dispose which propagates to disposeInternal of MapBrowserEventHandler where it unlistens avoiding the events to be called.

Expected behavior I am not sure which solution is preferable:

• Have events check if they have a target before checking the hasAttribute
• Making map.dispose needed for destroy (https://github.com/openlayers/openlayers/issues/10856#issuecomment-643601271 seems like it is not needed). I believe this is cleaner to avoid memory leaks everywhere. Could you tell me the reasoning to not make the dispose methods public?

Let me know what you prefer @tschaub

[ahocevar]

@EvertEt You are right. But we also want to handle the case where users change the map target, right? disposeInternal won’t be called in that case. So the best solution is to unregister all those listeners when the target is unset, and register them again when the target is set.

That said, I’m planning to release v6.4 this week. Do you think you’d be able to create a pull request? If not let me know, then I’ll take this on.

[EvertEt]

We have another similar error reported in MouseWheelZoom which doesn’t seem fixed by using map.dispose. We haven’t had time to look into it any further. Just posting this here in case someone else will look at creating a PR