Fix warning when running off() on databsae reference

Author: chrisbianca

lib/modules/database/reference.js

@@ -125,6 +125,7 @@ export default class Reference extends ReferenceBase {
   off(evt: string = '', origCB?: () => any) {
     const path = this.dbPath();
     const modifiers = this.query.getModifiers();
+    const modifiersString = this.query.getModifiersString();
     this.log.debug('ref.off(): ', path, modifiers, evt);
     return this.db.unstoreRef(this.uid).then(() => {
       return this.db.off(this.uid, path, modifiersString, modifiers, evt, origCB).then(subscriptions => {