Webgl dispose can throw

[Tyriar]

Rough repro:

1. Create terminal and open
2. Load addon
3. Detach element from DOM
4. Dispose addon

This line throws

xterm.js/addons/xterm-addon-webgl/src/WebglRenderer.ts

Line 111 in a385ab0

this._core.screenElement!.removeChild(this._canvas);

Ran into this when implementing unsplit which will reparent the terminal element. microsoft/vscode#121278