Fix an error where the main page gets blank when adding a server

[yuya-oc]

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Summary
Fix an error where the main page gets blank when adding a server.

When adding a server on the main page, it tries to get focus to the new webview. However probably its webContents object is not created yet, so null reference error happened for me.

Issue link
N/A

Original report: https://pre-release.mattermost.com/core/pl/53y7wdj3wtye3pnewpfajzuatc

Test Cases
Add a server from + button of the main page or [File/Mattermost ->Sign in to Another Server] menu item.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/822#artifacts

[jasonblais]

Thanks! Works for me on Mac.

[DHaussermann]

This is now working in latest windows build.

[yuya-oc]

Thanks!