One excuse that is commonly cited against performing contractual obligations is the impossibility of performance. Impossibility is not the same as impracticability, which is a doctrine that argues for excuse of non-performance of contractual obligations due to excessive and unreasonable levels of difficulty; impossibility is the more intuitive idea that a necessary condition of the performance of a contract is the existence of the item being sold or person rendering services. If a necessary person or item does not exist, it is impossible to actually perform the contract. Impossibility is recognized in many jurisdictions as an excuse of non-performance of a contractual obligation.
At an intuitive level, many people understand impossibility as an excuse for non-performance of a contract. People in informal or casual relationships will frequently excuse and forgive a promise made to them that is physically impossible to perform due to the nonexistence of an essential item or person. If one friend promises to another friend to buy a basket of apples from the store, and the store runs out of apples, then the friend who is the promisee is usually likely to excuse the inability to perform the promise due to the unavailability of apples at the store which the promisor friend visited.
The case Caldwell v. Taylor was an English court case from 1863 in which the court used the doctrine of impossibility in its opinion. Caldwell was the owner of a music hall and promised to rent the music hall to Taylor, a leasee, so that it could be used for hosting concerts and parties. However, before Taylor could make use of the property the building burned down, with neither party being responsible for the fire. Taylor sued Caldwell, but the court ruled that in this particular case, the existence of the property was an implied condition of the contract, and laid the groundwork for the doctrine of impossibility as an excuse for performance of a contract. This case has caused many people to argue that the existence of an essential item, or person, is a necessary condition of the contract that is implied when there is a meeting of the minds.
As your lawyer can explain, similar to a contracts lawyer an example of an essential person is a licensed professional such as an accountant, attorney, or broker. The sudden death or incapacity of such a professional would have enormous impacts on contracts that the professional is currently engaged in.