_get rc.context.<context with dash> fails

I have a context defined in my `taskrc`:

```
context.taskwarrior-fzf=project:taskwarrior-fzf
```

If I run `task context taskwarrior-fzf` the context changes - no problem.

If I run `task _get rc.context.taskwarrior-fzf` I expect to get: `project:taskwarrior-fzf` but I get an error:

```
'rc.context.taskwarrior-fzf' is not a DOM reference.
```

This error is both in branch `2.5.2` and `2.6.0`.