TypeError: Cannot read property 'length' of null for DataTable with headerCheckboxToggleAllPages true

[saverett]

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Current behavior
Before the DataTable's value property is initialized, the get allSelected() function is throwing a TypeError when the headerCheckboxToggleAllPages property is set to true.

Expected behavior
The get allSelected() function should also check to ensure the DataTable's value property is also defined (not null) before comparing its length to the selection length.

Minimal reproduction of the problem with instructions
Create a DataTable with paging, multiple select, and the headerCheckboxToggleAllPages property is set to true. Make sure the DataTable's value doesn't resolve right away.

What is the motivation / use case for changing the behavior?
The new headerCheckboxToggleAllPages DataTable option is missing a null check.

Please tell us about your environment:
Windows

• Angular version: 4.x

• PrimeNG version: 4.0.1

• Browser: All

• Language: All