How to reproduce:
1. Open the attached project "case_1117232-ToolbarIssue"
2. Make sure that Edit -> Preferences -> General -> "Editor Skin" is set to "Personal"
3. Open the user's custom Editor Window by going to Window -> UIElement - > Test
4. In the top left of the window click the dropdown with "abcd" written on it

Expected result: Dropdown box becomes pressed in (it has no elements)
Actual result: a yellow warning triangle texture is drawn on the box and a warning appears in the Console Window

Reproducible with: 2019.1.0b5, 2019.2.0a6
Could not test with: 2019.1.0a7 and older (UI element script have compilation errors)

Note: clicking the dropdown shows this warning: "Texture not found for path: Builtin Skins/LightSkin/Images/toolbar pulldown act uie.png
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)" in the Console Window. This warning is only shown the first time the dropdown is clicked