Steps to reproduce:

1. Create a new Unity project
2. Install the Terrain Tools package
3. Update the package to the latest v5.3.0
4. Observe the Console

Actual results: 
{code:java}
Unable to load the icon: 'Packages/com.unity.terrain-tools/Editor/Icons/TerrainOverlays/BrushSettingIcons/Opacity@2x.png'.
Note that either full project path should be used (with extension) or just the icon name if the icon is located in the following location: 'Assets/Editor Default Resources/Icons/' (without extension, since png is assumed)
UnityEditor.SceneView:OnEnable () (at /home/bokken/build/output/unity/unity/Editor/Mono/SceneView/SceneView.cs:1357)
{code}

Expected results: No warnings or errors in the Console

Reproducible with versions: 5.3.0 (6000.2.0b4)

Not Reproducible with versions: 5.3.0 (6000.0.50f1, 6000.1.6f1, 6000.2.0b3)

Tested on (OS): Ubuntu 22.04

Note:
- Didn't reproducible after restarting the project
- Sometimes it reproduces after creating/editing terrain