[Paint Brush in Raster mode not working]

Thanks you so much - awesome - all now working!!

[Paint Brush in Raster mode not working]

Hi,  I've been using Affinity Designer for a quite a while and never had any problems until I upgraded my iMac to High Sierra.  Now I can't see the paint effect when I use the raster brushes.  If I go up to "undo" it is undoing brush strokes that I have made but I can see nothing on screen.  I've been through the forums and checked the Assistant Manager to make sure that it is set to "rasterise layer and apply brush" but still no luck.  Any ideas?