Protect against an empty response (#3037)

This happened in some weird corner case while debugging

addon/webextension/selector/callBackground.js

@@ -4,10 +4,10 @@
 
 this.callBackground = function callBackground(funcName, ...args) {
   return browser.runtime.sendMessage({funcName, args}).then((result) => {
-    if (result.type === "success") {
+    if (result && result.type === "success") {
       return result.value;
-    } else if (result.type === "error") {
-      let exc = new Error(result.message);
+    } else if (result && result.type === "error") {
+      let exc = new Error(result.message || "Unknown error");
       exc.name = "BackgroundError";
       if ('errorCode' in result) {
         exc.errorCode = result.errorCode;
@@ -18,8 +18,8 @@ this.callBackground = function callBackground(funcName, ...args) {
       throw exc;
     } else {
       log.error("Unexpected background result:", result);
-      let exc = new Error(`Bad response type from background page: ${result.type || undefined}`);
-      exc.resultType = result.type || "undefined";
+      let exc = new Error(`Bad response type from background page: ${result && result.type || undefined}`);
+      exc.resultType = result ? (result.type || "undefined") : "undefined result";
       throw exc;
     }
   });