I assume that you are working on Adobe Reader DC desktop application and whenever you are trying to sign a document using "Fill & Sign" tool, the application crashes?
Does this happen with all the documents you sign or any specific one? Try using any other tool and see if the same thing happens?
In a mean while you may try the following-
1- Open Adobe Reader>help>Check for update. If no update, then-
2- Open Adobe Reader>help>Repair Installation. Still doesn't work, then-
3- Use cleaner tool to uninstall the application- Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs and reinstall it- Adobe Acrobat Reader DC Install for all versions
Let us know if none of the solution provided above works.