unnecessary_null_aware_assignments   
            
            Avoid null in null-aware assignment.
This rule is available as of Dart 2.0.0.
Rule sets: recommended, flutter
This rule has a quick fix available.
Details
AVOID null in null-aware assignment.
Using null on the right-hand side of a null-aware assignment effectively makes
the assignment redundant.
BAD:
var x;
x ??= null;
GOOD:
var x;
x ??= 1;
Usage
To enable the unnecessary_null_aware_assignments rule,
add unnecessary_null_aware_assignments under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - unnecessary_null_aware_assignments