TMP Dropdown visually corrupts or disappears when its own GameObject is assigned to "Caption Image" or "Placeholder" elements

Steps to reproduce:

1. Create a new project using Universal 3D template
2. In the Hierarchy, right-click > UI (Canvas) > Dropdown TextMeshPro
3. Import TMP Essentials if needed and select the created Dropdown
4. Find the "Caption Image" property and click on the object picker
5. Select "Dropdown" GameObject
6. Observe the Dropdown in Game view loses background
7. Try to CTRL + Z or select "None" to undo selection
8. Observe the behavior

Actual results: The Dropdown visually corrupts or completely disappears from the view. Pressing Undo or manually clearing the field fails to restore the correct visual state

Expected results: The Undo operation restores the previous visual state. Alternatively, the Object Picker filters out the Dropdown's own GameObject and internal components to prevent this unrecoverable state

Reproducible with versions: 2023.1.0a10, 6000.0.71f1, 6000.3.12f1, 6000.4.1f1, 6000.5.0b1, 6000.6.0a1

Tested on (OS): macOS 26.3 (M1 Max)

Notes:

• This applies to both Caption Image and Placeholder fields and selecting different GameObjects  in Object Picker has different effect
• This can be easy for a user to trigger accidentally because clicking the Object Picker opens the Scene tab by default and lists all the child components of the Dropdown
• Because Undo does not trigger a proper rebuild/refresh of the state, the user is forced to delete and recreate the Dropdown entirely