When i Create a TImageEnMView and don't fill it yet with images, but still click in an empty place and dragselect, i get an access violation. If i debugbreak i get at position 10842 in iemview.pas
// If region selection is beyond view of thumbnails (underneath) then region selection will be
// very slow because we will only increment in 1 pixel in routine below with thousands of calls to ImageAtPosWithCheckEvent
maxY := fIEMBitmap.GetImageInfo( GetImageCount() ............
It seems your speed optimisation doesn't handle empty selections yet.