To provide some background, this question is based on the game Crusader Kings 2 where a war over a territory can end if the defending ruler dies, even if the attacker is heavily favored in the "war score".
So as a secondary question, are there any examples of emperors/kings/dukes/counts who died while defending against an invasion and the attacker stopping their invasion?
Generally a war with a rival state in Medieval Europe was heavily focused around the monarch as the only power sufficient to enforce his will on a trans national stage. When a Monarch died it was not uncommon for a temporary peace settlement to develop either due to the removal of the personal antagonism of the individual being removed from the equation. Medieval Europe was not as stable as early modern Europe and particularly in Western Europe primogeniture (The automatic inheritance of the first born son) did not become the standard until the later middle ages. Many would find their priorities change as they had to secure their positions internally (Including senior aristocrats who owed their position to the previous Monarch) or establish their leadership as a new monarch.
In 1216 Prince Louis of France's invasion of England collapsed after the death of King John. The English baron's who had supported his invasion were opposed personally to King John rather than the idea of a Plantagenet King. Once he had died the reason behind their support for the invasion also died and so Louis had to give up and return to France.