Unparalleled problem with if-statement

Hello! I have a very strange problem because during game, once every three times "if" statement is not carried out and shows us error, that it can't find object.

This is Script: If player is on the sand texture:

 if(hit.collider.material.name == sandPhysicsMaterial.name + " (Instance)"){
     Debug.Log("Collider name ma nazwe: "+hit.collider.material.name+" zas gras material: "+sandPhysicsMaterial.name + " (Instance)");
     SetWheelStiffnessByGroundPhysic(2f);}

If player is on the grass texture:

 else if(hit.collider.material.name == grassPhysicsMaterial.name + " (Instance)"){
     Debug.Log("Collider name ma nazwe: "+hit.collider.material.name+" zas gras material: "+grassPhysicsMaterial.name + " (Instance)");
     SetWheelStiffnessByGroundPhysic(3f);}

If player is on the asphalt texture:

 (...) else {SetWheelStiffnessByGroundPhysic (1f);
 }

The script works on that when car will enter on the particular texture it will change the stiffness of the wheel: which works fine, but I do not know why when car enters the texture simply means a "Grass" is set in the new phisicsMaterial and at that moment that every 2-3 frame we've get an error. Can someone please help?