Converting null literal or possible null value to non-nullable type.

Possible null reference assignment.

Possible null reference argument for parameter 'expression' in 'Tuple<double, ErrorCode> Calculator.Evaluate(string expression, IStack stack)'.

Converting null literal or possible null value to non-nullable type.

Possible null reference assignment.

Possible null reference argument for parameter 'expression' in 'Tuple<double, ErrorCode> Calculator.Evaluate(string expression, IStack stack)'.

Dereference of a possibly null reference.

Nullability of reference types in type of parameter 'sender' of 'void CalculatorForm.button_Click(object sender, EventArgs e)' doesn't match the target delegate 'EventHandler' (possibly because of nullability attributes).

Dereference of a possibly null reference.