A potentially nullable type can't be used in an 'on' clause because it isn't valid to throw a nullable expressionDRT-W1548

The analyzer produces this diagnostic when the type following on in a catch clause is a nullable type. It isn't valid to specify a nullable type because it isn't possible to catch null (because it's a runtime error to throw null).

Example

The following code produces this diagnostic because the exception type is specified to allow null when null can't be thrown:

void f() {
  try {
    // ...
  } on FormatException? {
  }
}

Common fixes

Remove the question mark from the type:

void f() {
  try {
    // ...
  } on FormatException {
  }
}