Bug 1568150 - Stop spinning the event loop when destroying the inspector front. r=gl

InspectorFront.destroy isn't returning a promise. So the `await` in the toolbox
code is not relevant and is only spinning the event loop.
Removing it introduce races which should be fixed.
The toolbox buttons shouldn't try to unhighlight if the inspector isn't used.

Differential Revision: https://phabricator.services.mozilla.com/D39407

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

devtools/client/framework/components/ToolboxToolbar.js

@@ -273,8 +273,10 @@ class ToolboxToolbar extends Component {
         ref: "frameMenuButton",
         title: description,
         onCloseButton: async () => {
-          await toolbox.initInspector();
-          toolbox.highlighter.unhighlight();
+          // Only try to unhighlight if the highlighter has been started
+          if (toolbox.highlighter) {
+            toolbox.highlighter.unhighlight();
+          }
         },
       },
       this.createFrameList

devtools/client/framework/toolbox.js

@@ -3412,7 +3412,7 @@ Toolbox.prototype = {
 
       // Temporary fix for bug #1493131 - inspector has a different life cycle
       // than most other fronts because it is closely related to the toolbox.
-      await this._inspector.destroy();
+      this._inspector.destroy();
 
       this._inspector = null;
       this._highlighter = null;