late is not correct for _next in chain_of_responsibility

[mortezasojodi]

when you define _next late, for last Logger we get error (or any logger that we dont need _next ) because this variable is not initilized.
we must change it to Nullable inctance instead, like below:
Logger? _next;

[scottt2]

Hey @mortezasojodi ! Thanks for the report. I should be able to get to fixing this by the weekend or if you want to go ahead and file a PR with the fix. I am always happy to pull in contributor work!