type 'int' is not a subtype of type 'double'

static double checkDouble(dynamic value) {
    if (value is String) {
      return double.parse(value);
    } else {
      return value;
    }
  }
}

The problem seems from the last return value. You may need return value+.0.

You only need to add .toDouble() function to last returned value.

static double checkDouble(dynamic value) {
    if (value is String) {
      return double.parse(value);
    } else {
      return value.toDouble();
    }
  }
}

An idea would be to use num instead of int or double in this case.