Checking if legend destroy exists before clearing a chart on unmount

[shivangkumar]

When component unmounts before chart is completely rendered, in some race scenarios the legend object does not contain the destroy method and thus fails. This commit helps prevent the unmounting from breaking.

[Pauan]

@shivangkumar Thanks a lot for that!

However, I don't think it's quite correct. Charts aren't required to have a legend, and if they don't have a legend then your code will cause them to never be cleaned up, ever.

I'll need to discuss this with the team to figure out the best way to fix this.