Empty thumbnail after regenerating block

Details: <p><strong>Testing:</strong></p><p style="margin-left:40px">- Open Drive for which you know a corresponding block&nbsp;exists in the library.</p><p style="margin-left:40px">- Regenerate the block (the block should be replaced, and not just added.)</p><p style="margin-left:40px">- Close SkyCAD and reopen.</p><p style="margin-left:40px">- If the block is still there, we&#39;re good!</p><p style="margin-left:40px">NOTE:&nbsp; redo a couple of time with the same block to make sure...</p>