Expressions crach on id containe "_" or "."

[mmmcorpsvit]

Please add warning like this:

Warning! 
Id tag cannon contain "_" or "."!

i mean there

<textField id="user.First_Name" label="Enter your first name" value="super_user">    

this behavior crash expressions parser

[OndrejKunc]

I added _ as an allowed character in the identifier. The . cannot be used, because it can be used in the expressions to reference properties so there will be a conflict.
I added this log to the parser, whenever there is a prohibited character:

  developer.log(
    'Warning: Invalid identifier \'$id\' for element type $TFormElement in FormElementParser.\n'
    'Only alphanumerics characters and underscores are allowed and identifiers cannot start with a number.',
    level: 900, // 900 relates to Warning according to the package:logging
  );

It is already merged and published together with the null safety version, so I am closing it now.