v5.0.0 doesn't seem to work with null safety

[eggnstone]

Using

• Flutter 2.0.1
• Dart 2.12.0
• Mockito 5.0.0

This simple example

abstract class C
{
    int square(int x) <-- runtime error points here
    => x * x;
}

class MockC extends Mock
    implements C
{}

test('1', ()
{
    C c = MockC();
    when(c.square(1)); <-- runtime error points here
});

produces

type 'Null' is not a subtype of type 'int'

[srawlins]

Please read the NULL_SAFETY_README.