createStatement method Null safety

Implementation

static String createStatement() {
  String ret = '''
mutation createAStatement(\$input: CreateStatementInput!){
createStatement(
     input: \$input
  ){
statement{
  objectId
  ${DbFields.statementText}
  ${DbFields.statementPictureFile}{url}
  ${DbFields.statementYear}
  ${DbFields.statementMonth}
  ${DbFields.statementDay}
  ${DbFields.statementCorrectness}
  ${DbFields.statementMedia}
  ${DbFields.statementLanguage}
  ${DbFields.statementCategory}
  ${DbFields.statementMediatype}
  ${DbFields.statementAuthor}
  ${DbFields.statementLink}
  ${DbFields.statementRectification}
  ${DbFields.statementPictureCopyright}
  ${DbFields.statementFactcheckIDs}{
      edges{
        node{
          objectId
            ${DbFields.factText}
            ${DbFields.factAuthor}
            ${DbFields.factYear}
            ${DbFields.factMonth}
            ${DbFields.factDay}
            ${DbFields.factLanguage}
            ${DbFields.factMedia}
            ${DbFields.factLink}
            ${DbFields.factArchivedLink}
        }
      }
    }
  }
}
}
''';
  return ret;
}