Bug 1729925 - [devtools] Avoid leaking commands from toolbox. r=nchevobbe

Differential Revision: https://phabricator.services.mozilla.com/D125541

devtools/client/framework/toolbox.js

@@ -3876,6 +3876,8 @@ Toolbox.prototype = {
             this._win = null;
             this._toolPanels.clear();
             this.descriptorFront = null;
+            this.resourceCommand = null;
+            this.commands = null;
 
             // Force GC to prevent long GC pauses when running tests and to free up
             // memory in general when the toolbox is closed.