fix(backend): wait until quit to close backend

backend close listener was subscribed to the mainwindow close event. This would mean the backend is shut down whenever the window closes, which would break the app for any subsequent windows. Attaching backend closing to the lifespan of the app process with the 'will-quit' event makes more sense.

app/main.development.js

@@ -34,6 +34,13 @@ app.on('window-all-closed', () => {
   if (process.platform !== 'darwin') app.quit()
 })
 
+app.on('will-quit', () => {
+  if (backendProcess) {
+    backendProcess.close()
+    backendProcess = null
+  }
+})
+
 const installExtensions = () => {
   if (process.env.NODE_ENV === 'development') {
     const installer = require('electron-devtools-installer') // eslint-disable-line global-require
@@ -95,8 +102,6 @@ app.on('ready', () =>
 
       mainWindow.on('closed', () => {
         mainWindow = null
-        backendProcess.close()
-        backendProcess = null
       })
 
       if (process.env.NODE_ENV === 'development') {