fix: Close OAuth BrowserView only if present (#2313)

It seems our OAuth `BrowserView` can be undefined when it's time to
close it.
Although we're uncertain why, it could be Electron closes it already
when loading the redirect URL.

Anyway, we can check for its presence before trying to close it and
avoid errors.

gui/js/onboarding.window.js

@@ -115,7 +115,9 @@ module.exports = class OnboardingWM extends WindowManager {
   }
 
   closeOAuthView() {
-    this.win.removeBrowserView(this.oauthView)
+    if (this.oauthView) {
+      this.win.removeBrowserView(this.oauthView)
+    }
   }
 
   async create() {