src/vs/workbench/api/browser/mainThreadWebviews.ts/MainThreadWebviews/getWebview
class MainThreadWebviews:	private getWebview(handle: extHostProtocol.WebviewHandle): IWebview {
		const webview = this._webviews.get(handle);
		if (!webview) {
			throw new Error(`Unknown webview handle:${handle}`);
		}
		return webview;
	}