minor fix

lib/browser-polyfill/custom-browser-polyfill.js

@@ -89,7 +89,7 @@
 					addListener: listener => chrome.runtime.onInstalled.addListener(listener)
 				},
 				get lastError() {
-					return chrome.runtime.last;
+					return chrome.runtime.lastError;
 				}
 			},
 			storage: {