Bug 1485660 - Allows calling DebuggerClient.close twice in a row. r=jdescottes

MozReview-Commit-ID: uACM0VtJ5E

Differential Revision: https://phabricator.services.mozilla.com/D7456

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

devtools/shared/client/debugger-client.js

@@ -284,7 +284,9 @@ DebuggerClient.prototype = {
     this._eventsEnabled = false;
 
     const cleanup = () => {
-      this._transport.close();
+      if (this._transport) {
+        this._transport.close();
+      }
       this._transport = null;
     };