Un-toggling kills buffers in other tabs

When I have multiple buffers and tabs open, I get errors when exiting focus mode.

```
2 buffers wiped out
Error detected while processing function <SNR>51_ToggleFocusMode:
line    6:
E108: No such variable: "t:focusmode"
Press ENTER or type command to continue
```

 When I have time, I'll probably make a PR down the road since the issue looks like the variable is out of scope.