Cannot read property 'uri' of null fix

[jwchang0206]

There is a case that throw TypeError: Cannot read property 'uri' of null when I try to construct and remove the monaco editor in short amount of time in an web app. The setModelMarkers is in call stack already while the model is already removed.

[mention-bot]

@inspiredjw, thanks for your PR! By analyzing the history of the files in this pull request, we identified @alexandrudima, @egamma and @sandy081 to be potential reviewers.

[msftclas]

Hi @inspiredjw, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

It looks like you're a Microsoft contributor (Jeong Woo Chang). If you're full-time, we DON'T require a Contribution License Agreement. If you are a vendor, please DO sign the electronic Contribution License Agreement. It will take 2 minutes and there's no faxing! https://cla.microsoft.com.

TTYL, MSBOT;

[coveralls]

Coverage remained the same at 61.235% when pulling de65f69 on inspiredjw:patch-1 into 61328fd on Microsoft:master.

[alexdima]

Thank you for the contribution!