"Uncaught TypeError: Cannot read property 'time_point' of null" Error in Mask Tool

Hello author,

Thank you for providing such a useful tool for everyone to use.

When I use the Mask Tool to draw the mask, and press the Done button. I cannot add polygon. I can find the "**Uncaught TypeError: Cannot read property 'time_point' of null**" error message in the Console.

I can correct this error by modifying the following code:
https://github.com/CSAILVision/LabelMeAnnotationTool/blob/0e8b9f51ac528669dac27e7ddedb6aec8d1af056/annotationTools/js/handler.js#L313-L314

To:
`html_str += '' + anno.time_point[0] + '';`
`html_str += '' + anno.time_point[1] + '';`

Hope this finding can help everyone.