<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ncbi_geneid(self):
 """ Retrieve this feature's NCBI GeneID if it's present. NCBI GFF3 files contain gene IDs encoded in **Dbxref** attributes (example: `Dbxref=GeneID:103504972`). This function locates and returns the GeneID if present, or returns `None` otherwise. """

        values = self.get_attribute('Dbxref', as_list=True)
        if values is None:
            return None
        for value in values:
            if value.startswith('GeneID:'):
                key, geneid = value.split(':')
                return geneid
        return None