Death of one of two tenant parties during notice period

Scenario

Two parties A, B sign a rental contract in Germany. Only party A lives there, party B merely cosigns the contract, but neither makes any payments nor lives in the propery. Party B adopted party A as their child, this process is completed and is irrelevant I expect.

Party A desires to leave the property, party B is no longer capable of making decisions on their own due to dementia and so party C, procurer for B, agrees to cosign a notice with party A, because neither are interested in continuing to rent the property. Party C has all necessary paperwork to make legal decisions on behalf of B.

The landlord, a large corporate body, henceforth known as D, agrees to the notice and sends a notification explicitely stating their agreement towards contract termination after the notice period.

Party B dies during notice period. The dementia took its toll.

A informs D of these circumstances. A is no longer interested in looking for a new tenant during notice period to get out of the contract faster upon agreement with D, because A is busy with funeral preparations and decides to just pay the one remaining month of rent, despite no longer living in the property.

D ghosts.

There is no further communication from D after being notified of B’s death. The landlord, i.e. the corporate body and its representatives have gone completely silent.

Question

What exactly happens, when a tenant contract is cosigned by two parties, yet one of the parties dies during notice period? Can this in any way be cause to nullify the notice that was handed in?

I expect C will inherit from B, but this process is likely ongoing – even so, the notice has already been accepted and rightfully so when done, so is there a reason why the landlord is suddenly silent? Are they expecting continuing rental payments even though no one houses the property anymore and the notice period is over?

TL;DR Version

Two tenants cosign a rental contract. One of the tenants becomes disabled and notice is handed in by one tenant and a procurer of the other tentant. The landlord agrees. The tenant that became disabled before dies during notice period. Landlord ceases to respond upon learning this. The other tenant and the procurer are uncertain whether or not these special circumstances could for some reason nullify the handed in notice.