bindThis should check for undefined

[creage]

Description

When creating a new Chart, the bindThis method should check if the key is defined and not null. Otherwise this line fails:

const hasChild = Object.keys(fn[key]).length > 0; // this fails if the 'fn[key]' is null or undefined

Safe way

const hasChild = fn[key] != null && Object.keys(fn[key]).length > 0;

[netil]

Hi @creage, thanks for the report.
Would you like send PR(and its test case) for this?