Null safety

[netgfx]

Hello, will this be ported to null safety?

Thanks!

[KevinZhang19870314]

Hi, @sergiandreplace, flutter_rough is an awesome library, which is very friendly for hand-drawn widgets, so i am use it as dependency for my project wired_elements.

But its not null safety, I have to copy all the source code and do null safety myself, so could you please help to do null safety? As long as you do sound null safety, I will use this library as dependency instead of copying all the source code to my project.

Thanks!

[ark-sandbox]

Quick PR to do so #6