This is an old topic, but I have found a solution - hopefully anyone facing this issue going forward can use this.
1. Close FM if open
2. Go to Steam Library, right click on FM -> Properties.
3. Untick the 'Enable the Steam Overlay...'
4. Close Steam
5. Reopen Steam
6. Go to Steam Library, right click on FM -> Properties.
7. Tick the 'Enable the Steam Overlay...'
8. Reopen FM, editor should work now.