The employer does not control the I-485 application, since it is filed directly by you, not employer. In your case, the I-485 is based on the NIW I-140 which is the employer's filing. The employer can always withdraw or revoke the I-140 petition. If the I-140 petition has been approved, and the I-485 has been pending for 180 days, the employer can still revoke the I-140 petition approval. 

However, this does not prevent your case from being approved. Under the USCIS memo, an approved I-140 petition remains valid, once the I-485 application has been pending for 180 days, even if the employer requests the revocation of the I-140 petition.