I assume that this is a custom widget which you have developed on your side. Does the issue appear when clicking on the Edit button of the widget? Can you please let me know details about this widget and the designer of the widget?
In addition to this, can you please try to reproduce the issue and send me the sitefinity.log file located in the ~/App_Data folder of your 3.7 project? This may provide us more details about the issue.
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