Remix: Potentially should be constant but is not.

Question

I am trying to execute a contract provided as a tutorial for Remix at the following link: https://gangachris.com/posts/a-look-into-remix-ide/ The contract is:

 pragma solidity ^0.4.20;

contract Cars{
   struct Car{
      bytes32 make;
      unint year;
   }

   mapping(address => Car[]) public carOwners;

   function registerCar(bytes32 _make, uint r) public {
      carOwners[msg.sender].push(Car({ make: _make,
                                       year: yr}));
   }
   function changeOwnership(address toOwner, uint index) public returns (bool){
      if(carOwners[msg.sender].length == 0) {
         return false;
      }
      if(carOwners[msg.sender].length > index + 1) {
         return false;
      }
      carOwners[toOwner].push(carOwners[msg.sender][index]);
    }
    function getCarMake(address owner, uint index) public view returns (bytes32 carMake) {
       if(carOwners[owner].length == 0) {
          return;
       }
       if(carOwners[owner].length > index + 1){
          return;
       }
       carMake = carOwners[owner][index].make;
     }
}

I found by search that it’s a Remix problem and not associated with Solidity. Some body please guide me.

Zulfi.