Bug 1595875, fix handling of return value of FormAutoComplete:GetSelectedIndex that was incorrectly removed by 1573836, r=MattN

Differential Revision: https://phabricator.services.mozilla.com/D52720

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

toolkit/actors/AutoCompleteChild.jsm

@@ -182,9 +182,20 @@ class AutoCompleteChild extends JSWindowActorChild {
     // time it changes because not every action that can change the
     // selectedIndex is trivial to catch (e.g. moving the mouse over the
     // list).
-    return Services.cpmm.sendSyncMessage("FormAutoComplete:GetSelectedIndex", {
-      browsingContext: this.browsingContext,
-    });
+    let selectedIndexResult = Services.cpmm.sendSyncMessage(
+      "FormAutoComplete:GetSelectedIndex",
+      {
+        browsingContext: this.browsingContext,
+      }
+    );
+
+    if (
+      selectedIndexResult.length != 1 ||
+      !Number.isInteger(selectedIndexResult[0])
+    ) {
+      throw new Error("Invalid autocomplete selectedIndex");
+    }
+    return selectedIndexResult[0];
   }
 
   get popupOpen() {