When a candidate of any political party has filed a nominating petition for such office and has been nominated for said office by said party, he shall not be permitted to withdraw unless he has removed from the state, or has become physically unfit. No vacancy shall be filled by the state central committees except for the causes and as herein specified: Provided, That this prohibition shall not be construed to prohibit the withdrawal of any candidate who has been nominated without having filed a nominating petition and whose name has been written or placed on the ballot of any political party.