fix(wallet): dereference window after closing

Dereference the main window after it has been closed in order to prevent
messages from being set to it after it when it no longer exists.

app/lib/zap/controller.js

@@ -112,6 +112,12 @@ class ZapController {
         this.mainWindow.hide()
       }
     })
+
+    // Dereference the window object, usually you would store windows in an array if your app supports multi windows,
+    // this is the time when you should delete the corresponding element.
+    this.mainWindow.on('closed', () => {
+      this.mainWindow = null
+    })
   }
 
   // ------------------------------------

app/main.dev.js

@@ -109,7 +109,9 @@ app.on('ready', () => {
       event.preventDefault()
       zap.terminate()
     } else {
-      zap.mainWindow.forceClose = true
+      if (zap.mainWindow) {
+        zap.mainWindow.forceClose = true
+      }
     }
   })