Flutter project Dart Error

When trying to build the web app using flutter in android studio i am getting a ton of errors that say.

Error: the argument type 'list<onlinecartmodel>?' Cant be assigned to the parameter type 'list<onlinecartmodel>' because 'list<onlinecartmodel>?' is nullable and
'list<onlinecartmodel>' isn't.

Ive attached screenshots of the full error. I need someone to help me reolve this as well as another error.