src/vs/workbench/contrib/customEditor/common/customEditorModelManager.ts/CustomEditorModelManager/add
class CustomEditorModelManager:	public add(resource: URI, viewType: string, model: Promise<ICustomEditorModel>): Promise<IReference<ICustomEditorModel>> {
		const key = this.key(resource, viewType);
		const existing = this._references.get(key);
		if (existing) {
			throw new Error('Model already exists');
		}

		this._references.set(key, { viewType, model, counter: 0 });
		return this.tryRetain(resource, viewType)!;
	}