Bug 1522994 - Resolves crash when using Touch Bar inputs out-of-context. r=mikedeboer

Differential Revision: https://phabricator.services.mozilla.com/D17695

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

browser/components/touchbar/MacTouchBar.js

@@ -232,16 +232,21 @@ class TouchBarHelper {
   }
 
   getTouchBarInput(inputName) {
+    // inputName might be undefined if an input's context() returns undefined.
+    if (!inputName || !kBuiltInInputs.hasOwnProperty(inputName)) {
+      return null;
+    }
+
     if (typeof kBuiltInInputs[inputName].context == "function") {
       inputName = kBuiltInInputs[inputName].context();
     }
 
-    let inputData = kBuiltInInputs[inputName];
-
-    if (!inputData) {
+    if (!inputName || !kBuiltInInputs.hasOwnProperty(inputName)) {
       return null;
     }
 
+    let inputData = kBuiltInInputs[inputName];
+
     let item = new TouchBarInput(inputData);
 
     // Async l10n fills in the localized input labels after the initial load.