unwrapErrOrNull abstract method

F? unwrapErrOrNull()

Returns the err value if Result is Err, otherwise returns null. This can be used to determine is Ok or is Err, since when the failure type is not nullable, so a returned null value means this is not an Err. Same as "err()" in Rust, but "err" is a field name here

Implementation

F? unwrapErrOrNull();