made layers optional. check if chunk attr exists.

[joshua-gould]

[mojaveazure]

Hi Joshua,

Thanks for catching this! For a number of reasons, we try to have everything merge first into the develop branch before eventually rolling into master. I changed the base branch here but that's caused some merge conflicts since the changes were made on your fork of master. Might be easiest to just copy the changes over to develop and open a new PR?