Crash when deselecting string property in Custom Types Editor #3465

Describe the bug In the Custom Types Editor window, when deselecting a string property member value by clicking on another custom Type, Tiled always crash.

To Reproduce Steps to reproduce the behavior:

1. Open the Custom Types Editor window (View > Custom Types Editor)
2. Click on Add Enum and name it Enum1
3. Click on Add Class and name it Class1
4. Click on Add Member (for Class1)
5. Choose string as type, name it as you want
6. Click on the empty field of the string member
7. Click on the Enum1 in the Custom Types list
8. Crash

Expected behavior No crash.

Media Video of the crash repro steps in my project: tiled_crash_custom_types.webm

Specifications:

• OS: Linux Manjaro (AMD Radeon RX 5600 XT navi10, LLVM 14.0.6, DRM 3.44, 5.17.15-1-MANJARO)
• Tiled Version: 1.9.0, 1.9.1 (from AUR: https://archlinux.org/packages/community/x86_64/tiled/)

Notes:

• Apologies if it's a duplicate, I did a quick search of open and closed issues but didn't see a mention of the Custom Types Editor.
• Seems to only happen when deselecting by clicking on an Enum, even if empty.