Fixed one minor issue related to "Reassign objects to"

I have fixed one minor issue when user right click on any login role and select "Reassign/Drop Owned..." context menu. When user click on "OK" button, but there is no role in the combo box. Attached is the screenshot of the error.

I have fixed the issue by disabling the "OK" button if there is no role in the combo box. Attached is the patch file, please review it and if it looks good then can you please commit it. 
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