qt: Call setParent() in the parent's context

setParent(parent) calls sendEvent(parent, QChildEvent) that implies
running in the thread which created the parent object.

src/qt/walletcontroller.cpp

@@ -131,7 +131,11 @@ WalletModel* WalletController::getOrCreateWallet(std::unique_ptr<interfaces::Wal
     WalletModel* wallet_model = new WalletModel(std::move(wallet), m_client_model, m_platform_style, nullptr);
     // Handler callback runs in a different thread so fix wallet model thread affinity.
     wallet_model->moveToThread(thread());
-    wallet_model->setParent(this);
+    // setParent(parent) must be called in the thread which created the parent object. More details in #18948.
+    GUIUtil::ObjectInvoke(this, [wallet_model, this] {
+        wallet_model->setParent(this);
+    }, GUIUtil::blockingGUIThreadConnection());
+
     m_wallets.push_back(wallet_model);
 
     // WalletModel::startPollBalance needs to be called in a thread managed by