Hi Samir Vaidya,
This error may occur if you have tried to create a custom designer view for the Feather Image Gallery Widget and Sitefinity is not able to find the designer. Please refer to the following post for more details about this issue and how to solve it:
You may also refer to the following article for more details about creating custom designer views:
If you have not created a custom designer view and the issue reproduces with the default designer views of the Image Gallery widget, what I can suggest is to delete the files under the Logs folder (or copy the files to another location for backup) and then try to reproduce the issue again by opening the designer of the widget. After that please check if there are any log files under the Logs folder and send the files to inspect them. They may contain more details about the issue and waht is causing it.
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