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.

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