Trying to select from a number panics in interactive mode

Running:
```sh
echo 123 > file
t -i file
# type "n@0"
```
causes `t` to crash with the message: `panicked at src/interpreter.rs:83:19: context should have root value`.

Just running `t n@0 file` normally, yields `Error: cannot select from number`.

It would be great if that error could just be displayed instead of crashing :)