fix: Close OAuth BrowserView only if present

  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.

Author: taratatach

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() {