I had never used that widget before, so today I tried it. Steps:
1. Unlock widgets
2. "Add Widget"
3. Searched for "Trash", found it, double-clicked it. The icon appeared on my panel.
4. Opened Dolphin, selected 3 files in my home folder, right-clicked and chose "Move to Trash".
5. Observed the trash icon change to show something at the top.
6. Clicked the trash icon to open it.
7. Observed the 3 files that I had deleted from my home folder.
So, I cannot replicate your issue -- does the included trash function in Dolphin work correctly?