fix for JSON Parse error when using dash_ag_grid and dash pages

[BSd3v]

Fixes #299.

• testing for grid destroyed before trying to get the state

[gvwilson]

@archmoj could you please have a look at this (or let us know that you don't feel you have enough context to review)?

[archmoj]

@BSd3v Thanks very much for the PR. It's looking good.
Could you add a test to lock down this bug from happening again?

@gvwilson I am not familiar with this repository.
By looking at the list of contributors @AnnMarieW, @ndrezn and @Farkites may help approving this PR.

BTW. I was wondering perhaps this check could/should be moved to be inside getColumnState as an early return to avoid duplicates and simplify this PR as well.

[BSd3v]

Hello @archmoj,

getColumnState is not something that this repository owns as it comes from the underlying AG Grid. While yes they could do a return, they provide the warning that you are calling a function against a destroyed grid.

[BSd3v]

Hello @gvwilson this issue can crop up at any point the grid is destroyed, this doesn't just apply to when using pages. Although it's a lot easier to test when using pages. 😁