src/vs/workbench/api/common/extHostCustomEditors.ts/ExtHostCustomEditors/getCustomEditorProvider
class ExtHostCustomEditors:	private getCustomEditorProvider(viewType: string): vscode.CustomEditorProvider {
		const entry = this._editorProviders.get(viewType);
		const provider = entry?.provider;
		if (!provider || !isCustomEditorProviderWithEditingCapability(provider)) {
			throw new Error('Custom document is not editable');
		}
		return provider;
	}