[MAGNAT2645]

Just add a check before GetEntityClassname (line 437)
Like this:

    new iWeaponEnt = GetEntPropEnt(attacker, Prop_Send, "m_hActiveWeapon");
    if (iWeaponEnt != -1 && IsValidEntity(iWeaponEnt)) {
        GetEntityClassname(iWeaponEnt, szClassName, sizeof(szClassName));
        if (StrEqual(szClassName, "tf_weapon_knife") && (damagetype & DMG_CRIT == DMG_CRIT))
        {
            //Backstab detected.
            if (IsPlayerInTank[victim]){
                damage = 200.0;
                action = Plugin_Changed;
            }
        }
        if (IsPlayerInTank[attacker] && (damagetype & DMG_CRIT == DMG_CRIT)){
            if (Supercrits[attacker]){
                damage = damage * 2.5;
                action = Plugin_Changed;
            }
        }
        if (IsPlayerInTank[victim] && !IsPlayerInTank[attacker]){
            if (Supercrits[victim] && !(damagetype & TF_DMG_MELEE == TF_DMG_MELEE)){
                damage = damage * 0.55;
                action = Plugin_Changed;
            }
            if (FiresHeavyLasers[victim] && GetClientHealth(victim) >= 2000){
                damage = damage * 0.75;
                action = Plugin_Changed;
            }
            if (InvisiTank[victim] && damage >= 200.0){
                if (TF2_IsPlayerInCondition(victim, TFCond:110)){
                    TF2_RemoveCondition(victim, TFCond:110);
                }
            }
        }
    }
	
	
    return action;