Access of possibly undefined property hitBox through a reference with static type

Hello i have this issue and its to do with this line

if (this.hitBox.hitTestObject(child.bodyBox)) {         <-------- this lines causes error
// hit action [vec POS, damage, atk type, effect (0~4), effect pow, push, fatality, flyX, flyY, flyZ]
child.hitCalculate(child.x - this.x, _damage, 2, 1, 1.5, 5 + Math.random(), 1);