*ngIf on a mat-step breaks the stepper if the expression becomes false from a later step

[adamdport]

Bug, feature request, or proposal:

bug

What is the expected behavior?

I can remove a step at any time without the stepper crashing

What is the current behavior?

If an earlier step is removed while a later step is active, the current step disappears. If you manually click a still visible previous step, we get an error Error: Cannot set property 'interacted' of undefined

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-fm21qe

1. toggling step 2 from step 1 works fine.
2. toggling step 2 from step 3 breaks step 3
3. after toggling from step 3, navigation to step 1 emits an error

What is the use-case or motivation for changing an existing behavior?

We can remove later steps using *ngIf without issue, so one would presume they could remove a step at any time. If it's decided that removing a previous step doesn't make sense, this should at least fail gracefully.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

6.2.0

Is there anything else we should know?

[adamdport]

@crisbeto is this resolved by #9132? Or is it a different issue? That PR handles tabs, not sure if there's anything that's reused in there or not.

[crisbeto]

There's #11813 which handles this one.

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