Fixed javascript error when window closed (OSX)

This https://github.com/zulip/zulip-electron/issues/32

app/main/index.js

@@ -210,6 +210,15 @@ app.on('ready', () => {
 });
 
 ipc.on('new-domain', (e, domain) => {
-	mainWindow.loadURL(domain);
+	//mainWindow.loadURL(domain);
+	if (!mainWindow) {
+	 	mainWindow = createMainWindow();
+	 	mainWindow.loadURL(domain);
+	 } else if (mainWindow.isMinimized()){
+	 	mainWindow.loadURL(domain);
+	 	mainWindow.show();
+	 } else {
+	 	mainWindow.loadURL(domain);
+	 }
 	targetLink = domain;
 });