Fix maintenance of grammars for copied editors

src/null-grammar.js

@@ -1,4 +1,13 @@
+/** @babel */
+
+import {Disposable} from 'event-kit'
+
 module.exports = Object.freeze({
   name: 'Null Grammar',
-  scopeName: 'text.plain'
+  scopeName: 'text.plain',
+  onDidUpdate (callback) {
+    return new Disposable(noop)
+  }
 })
+
+function noop () {}