"Cannot read property 'btn' of null" exception occurs randomly

[youssmak]

Ionic version: (check one with "x")
[ ] 1.x (For Ionic 1.x issues, please use https://github.com/ionic-team/ionic-v1)
[ ] 2.x
[X] 3.x

I'm submitting a ... (check one with "x")
[x] bug report
[ ] feature request
[ ] support request => Please do not submit support requests here, use one of these channels: https://forum.ionicframework.com/ or http://ionicworldwide.herokuapp.com/

Current behavior:
Cannot read property 'btn' of null exception occurs randomly

Expected behavior:
avoid raising this exception by checking if tab is defined

Related code:

https://github.com/ionic-team/ionic/blob/63fd63e72e8cf44767ec8ca2d6ba0e6b9f071317/src/components/tabs/tab-highlight.ts#L21

[ionitron-bot]

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