Merge bitcoin/bitcoin#36176: wallet: avoid a crash when creating a wallet with -nosettings

### src/interfaces/chain.h
@@ -358,6 +358,9 @@ class Chain
     //! support for writing null values to settings.json.
     //! Depending on the action returned by the update function, this will either
     //! update the setting in memory or write the updated settings to disk.
+    //! Returns false if the update function returned no action, or if the
+    //! settings could not be written to disk, including when settings are
+    //! disabled with -nosettings. In-memory changes are kept either way.
     virtual bool updateRwSetting(const std::string& name, const SettingsUpdate& update_function) = 0;
 
     //! Replace a setting in <datadir>/settings.json with a new value.

### src/node/interfaces.cpp
@@ -843,7 +843,7 @@ class ChainImpl : public Chain
         });
         if (!action) return false;
         // Now dump value to disk if requested
-        return *action != interfaces::SettingsAction::WRITE || args().WriteSettingsFile();
+        return *action != interfaces::SettingsAction::WRITE || (args().GetSettingsPath() && args().WriteSettingsFile());
     }
     bool overwriteRwSetting(const std::string& name, common::SettingsValue value, interfaces::SettingsAction action) override
     {

### test/functional/wallet_startup.py
@@ -64,6 +64,35 @@ def test_load_unwritable_wallet(self, node):
         # Reset directory permissions for cleanup
         dir_path.chmod(original_dir_perms)
 
+    def test_disabled_settings(self, node):
+        self.log.info("Test wallet startup preferences with dynamic settings disabled")
+        load_message = "Wallet load on startup setting could not be updated, so wallet may not be loaded next node startup."
+
+        settings_path = node.chain_path / "settings.json"
+        settings_before = settings_path.read_bytes()
+        self.restart_node(0, extra_args=["-nosettings"])
+        assert_equal(node.listwallets(), [''])
+
+        assert_equal(node.createwallet(wallet_name="no_settings", load_on_startup=True), {"name": "no_settings", "warnings": [load_message]})
+        assert_equal(set(node.listwallets()), {'', 'no_settings'})
+
+        # Leaving the startup preference unchanged does not warn, and the wallet remains usable.
+        assert_equal(node.unloadwallet(wallet_name="no_settings"), {})
+        assert_equal(node.loadwallet(filename="no_settings"), {"name": "no_settings"})
+        assert_equal(node.get_wallet_rpc("no_settings").getwalletinfo()["walletname"], "no_settings")
+
+        assert_equal(node.loadwallet(filename="w2", load_on_startup=True), {"name": "w2", "warnings": [load_message]})
+        assert_equal(set(node.listwallets()), {'', 'no_settings', 'w2'})
+
+        assert_equal(node.unloadwallet(wallet_name="no_settings", load_on_startup=False), {"warnings": ["Wallet load on startup setting could not be updated, so wallet may still be loaded next node startup."]})
+        assert_equal(set(node.listwallets()), {'', 'w2'})
+        self.stop_node(0)
+        assert_equal(settings_path.read_bytes(), settings_before)
+
+        # Re-enabling settings restores the original startup preferences.
+        self.start_node(0)
+        assert_equal(set(node.listwallets()), {'w2', 'w3'})
+
     def run_test(self):
         self.log.info('Should start without any wallets')
         assert_equal(self.nodes[0].listwallets(), [])
@@ -94,6 +123,7 @@ def run_test(self):
         assert_equal(set(self.nodes[0].listwallets()), set(('w2', 'w3')))
 
         self.test_load_unwritable_wallet(self.nodes[0])
+        self.test_disabled_settings(self.nodes[0])
 
 if __name__ == '__main__':
     WalletStartupTest(__file__).main()