Describe the bug
An unhandled System.InvalidOperationException is being thrown while one or more selections in the Documents DataGridView are being processed and a find or unchecking one of the selections is attempted.

To Reproduce
Steps to reproduce the behavior:

1. Select (check) one or more documents in the Documents DataGridView.
2. Perform any operation on the selections.
3. While the operation is running, attempt to perform a Find or uncheck one of the selections in the Documents DataGridView.
4. See error

Expected behavior
The Find menu item and tool bar button should be disabled while an operation is being performed on one or more selected (checked) documents in the Documents DataGridView. In addition, it should not be possible to check/uncheck any check boxes in the Documents DataGridView also.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10 22H2
• Browser: N/A
• Browser Version: N/A
• PDFKeeper Version: 11.2.1 (older versions are also impacted)

Database (please complete the following information):

• DBMS: N/A
• Version: N/A

Additional context
PDFKeeper.log