Reliably dispose of ConsoleWidget when entering play mode

[ElmarTalibzade]

Pull Request

Summary

If you had "Clear Console On Play" enabled, you'd get the below, public facing error:

Error calling event 'scene.startplay' on 'Editor.ConsoleWidget'

But this is the actual error that's being tossed around:

QPlainTextEdit was null when calling clear

I believe it's caused because ConsoleWidget wasn't being propely disposed of.

Motivation & Context

Fixes: #10440

Implementation Details

The fix was straightforward, I made sure that ConsoleWidget singleton instance was being properly disposed when OnDestroyed callback is called.

I snooped around and got inspired by how it was already done in the codebase - mainly in MainAssetBrowser (source) and SceneNetworkSystem (source).

I also spotted that Sandbox.ActionGraphs using directive was unused so I nuked it.

Screenshots / Videos (if applicable)

This is the error message people would be seeing in master at the present.

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂