Bug 1744556 - [devtools] Avoid exception about server sent events when opening the browser toolbox. r=bomsy

Differential Revision: https://phabricator.services.mozilla.com/D132988

devtools/shared/commands/resource/legacy-listeners/server-sent-events.js

@@ -7,6 +7,10 @@
 const ResourceCommand = require("devtools/shared/commands/resource/resource-command");
 
 module.exports = async function({ targetFront, onAvailable }) {
+  if (!targetFront.getTrait("isBrowsingContext")) {
+    // This resource is only available on browsing-context targets.
+    return;
+  }
   const eventSourceFront = await targetFront.getFront("eventSource");
   eventSourceFront.startListening();