This has happened occasionally, including today. I took note, to share in case anyone else gets stuck with no apparent way out. Ref. 2018 SP3 installation.
In a drawing, I Edit Sheet Format. While doing so, today I deleted an unused block from the tree. Other actions have also caused the following error. After doing something 'unexpected', like deleting an unused block, then the icon disappeared in the upper right to close edit sheet format and return to the drawing contents. The Close (file) icon is still there, but the space below it is empty.
When this icon disappears, here's what to do. It is un-intuitive. Right click the sheet, either in display or in the tree. Click "Edit Sheet". Even though you are already editing sheet format, selecting "Edit Sheet" from menu will then exit editing sheet format.
It would make more sense if it said Stop Editing Sheet or something. No worries.