The physics
At a null point, all the forces add up to zero. A test charge there feels no net force.
k q₁ / x² = k q₂ / (d − x)²
x : (d − x) = √q₁ : √q₂
Like charges: the null point lies between them, nearer the smaller charge.
Unlike charges: it lies outside, beyond the smaller charge.
A medium with dielectric constant K divides every force by K. So the null point does not move.
Gravity only pulls. So two masses always have a null point between them.