Don't assume that the scrollMask is present when closing the md-dialog

[janvandenberg]

Fixed an issue where md-dialog contains md-autocomplete elements. These elements remove the scroll mask before the dialog is closed, resulting in an exception when closing the dialog. This a symptom fix. I don't assume that the mask is still there before I try to delete it. But now the dialog closed properly and no errors.

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[janvandenberg]

I signed it!

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[ThomasBurleson]

@janvandenberg - lgtm