not generate model file in dart 2.13

✖ + error_unexpected Could not format because the source could not be parsed:

line 4, column 3 of .: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
╷
4 │ List? friends;
│ ^^^^
╵
line 4, column 3 of .: Expected to find ';'.
╷
4 │ List? friends;
│ ^^^^
╵
line 4, column 17 of .: A function body must be provided.
╷
4 │ List? friends;
│ ^^^^^^^
╵
line 4, column 17 of .: Variables must be declared using the keywords 'const', 'final', 'var' or a type name.
╷
4 │ List? friends;
│ ^^^^^^^
╵
line 4, column 7 of .: The string '?' isn't a user-definable operator.
╷
4 │ List? friends;
│ ^
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line 4, column 7 of .: Methods must have an explicit list of parameters.
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4 │ List? friends;
│ ^
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line 4, column 7 of .: Operator declarations must be preceded by the keyword 'operator'.
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4 │ List? friends;
│ ^
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line 4, column 8 of .: Types parameters aren't allowed when defining an operator.
╷
4 │ List? friends;
│ ^^^^^^^^
╵