Transformation Event Handler Guidance

Hi,

I have created a transformation event handler where it throws an error when an user tries to "Clear" file/s. in Import step. Now, the error comes when user tries to delete his/her file or "All" files. I only wanted it to throw error when user (mistakenly) selects "All" imported files and click on "Clear"

Is there a way to achieve this by some conditions or checks? 

Thanks in advance,

PM