Null pointer check needed as of rust 1.86

See tauri-apps/plugins-workspace#2452

Author: jf908

src/windows_single.rs

@@ -95,6 +95,11 @@ unsafe extern "system" fn single_instance_window_proc(
     lparam: LPARAM,
 ) -> LRESULT {
     let data_ptr = GetWindowLongPtrW(hwnd, GWL_USERDATA) as *mut Box<SingleInstanceCallback>;
+
+    if data_ptr.is_null() {
+        return DefWindowProcW(hwnd, msg, wparam, lparam);
+    }
+
     let callback = &mut *data_ptr;
 
     match msg {