az interactive crashes when changing scope

### **This is autogenerated. Please review and update as needed.**

## Describe the bug
az interactive crashes when changing scope

**Command Name**
`az interactive`

**Errors:**
```
'NoneType' object has no attribute 'in_tree'
Traceback (most recent call last):
python3.8/site-packages/knack/cli.py, ln 206, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 608, in execute
raise ex
cli/core/commands/__init__.py, ln 666, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 659, in _run_job
six.reraise(*sys.exc_info())
...
interactive/azext_interactive/azclishell/app.py, ln 520, in _special_cases
continue_flag, cmd = self.handle_scoping_input(continue_flag, cmd, cmd_stripped)
interactive/azext_interactive/azclishell/app.py, ln 599, in handle_scoping_input
if self.completer.command_tree.in_tree(tree_path):
AttributeError: 'NoneType' object has no attribute 'in_tree'
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az interactive`
- `%% ad`

## Expected Behavior

## Environment Summary
```
Linux-4.19.76-linuxkit-x86_64-with
Python 3.8.1
Shell: bash

azure-cli 2.0.81 *

Extensions:
aks-preview 0.4.27
webapp 0.2.24
subscription 0.1.3
interactive 0.4.3

```
## Additional Context