Merge bitcoin-core/gui#864: Crash fix, disconnect numBlocksChanged() signal during shutdown

71656bdfaa gui: crash fix, disconnect numBlocksChanged() signal during shutdown (furszy)

Pull request description:

  Aiming to fix bitcoin-core/gui#862.

  The crash stems from the order of the shutdown procedure:
  We first unset the client model, then destroy the wallet controller—but we leave
  the internal wallet models (`m_wallets`) untouched for a brief period. As a result,
  there’s a point in time where views still have connected signals and access to
  wallet models that are not connected to any wallet controller.
  Now.. since the `clientModel` is only replaced with nullptr locally and not destroyed
  yet, signals like `numBlocksChanged` can still emit. Thus, when wallet views receive
  them, they see a non-null wallet model ptr, and proceed to call backend functions
  from a model that is being torn down.

  As the shutdown procedure begins by unsetting `clientModel` from all views. It’s safe
  to ignore events when `clientModel` is nullptr.

ACKs for top commit:
  maflcko:
    lgtm ACK 71656bdfaa
  pablomartin4btc:
    re-ACK 71656bdfaa
  hebasto:
    ACK 71656bdfaa, I have reviewed the code and it looks OK.

Tree-SHA512: e6a369c40aad8a5a3da64e92daa10250006f60c53feef353a5580e1bdb17fe8e1ad102abf5419ddeff1caa703b69ab634265ef3b9cfef87e9304f97bfdd2c4aa

src/qt/sendcoinsdialog.cpp

@@ -855,6 +855,10 @@ void SendCoinsDialog::updateCoinControlState()
 }
 
 void SendCoinsDialog::updateNumberOfBlocks(int count, const QDateTime& blockDate, double nVerificationProgress, SyncType synctype, SynchronizationState sync_state) {
+    // During shutdown, clientModel will be nullptr. Attempting to update views at this point may cause a crash
+    // due to accessing backend models that might no longer exist.
+    if (!clientModel) return;
+    // Process event
     if (sync_state == SynchronizationState::POST_INIT) {
         updateSmartFeeLabel();
     }