Bug 1588728 - Add a check for this._highlightedNodeFront in _hideBoxModel. r=pbro

Differential Revision: https://phabricator.services.mozilla.com/D49632

--HG--
extra : moz-landing-system : lando

devtools/client/inspector/markup/markup.js

@@ -731,6 +731,10 @@ MarkupView.prototype = {
    *         requests queued up
    */
   _hideBoxModel: function(forceHide) {
+    if (!this._highlightedNodeFront) {
+      return Promise.resolve();
+    }
+
     return this._highlightedNodeFront.highlighterFront
       .unhighlight(forceHide)
       .catch(this._handleRejectionIfNotDestroyed);