src/vs/workbench/api/common/extHostCustomEditors.ts/ExtHostCustomEditors/getCustomDocumentEntry
class ExtHostCustomEditors:	private getCustomDocumentEntry(viewType: string, resource: UriComponents): CustomDocumentStoreEntry {
		const entry = this._documents.get(viewType, URI.revive(resource));
		if (!entry) {
			throw new Error('No custom document found');
		}
		return entry;
	}