You can parse a string into an integer with int.parse(). For example:

var myInt = int.parse('12345');
assert(myInt is int);
print(myInt); // 12345

Note that int.parse() accepts 0x prefixed strings. Otherwise the input is treated as base-10.

You can parse a string into a double with double.parse(). For example:

var myDouble = double.parse('123.45');
assert(myDouble is double);
print(myDouble); // 123.45

parse() will throw FormatException if it cannot parse the input.

If the value was parsed from JSON, Dart knows the value is a double and the error is telling me that the value is of type double and the convert expected type string.

So the error is much more clear once I understood!

Remove the quotes from

 var long2 = double.parse('$group1');

to

 var long2 = double.parse($group1);

You can also use

var long2 = double.tryParse($group1);

or to also accept numbers without fractions

var long2 = num.tryParse($group1)?.toDouble();

to not get an exception when the string in $group1 can not be converted to a valid double.