VirtualScroll Can't read property length of null

[ionitron-bot]

Original issue by @aggarwalankush on 2017-04-29T00:41:41Z

Ionic version: (check one with "x")
[ ] 1.x
[ ] 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:
I got Null exception at https://github.com/driftyco/ionic/blob/master/src/components/virtual-scroll/virtual-scroll.ts#L601

Expected behavior:
Check for null before calling length function

Steps to reproduce:

Don't know how to reproduce. Got it only a few times.

Related code:

insert any relevant code here

Other information:

Ionic info: (run ionic info from a terminal/cmd prompt and paste output below):

Your system information:

Cordova CLI: 6.5.0 
Ionic Framework Version: 3.1.1
Ionic CLI Version: 2.2.3
Ionic App Lib Version: 2.2.1
Ionic App Scripts Version: 1.3.6
ios-deploy version: 1.9.1 
ios-sim version: 5.0.13 
OS: macOS Sierra
Node Version: v7.3.0
Xcode version: Xcode 8.3.2 Build version 8E2002