Reproduction steps:
1. Create a new Unity project.
2. Create and open the iOS Xcode project.
3. Go to the Xcode Build Settings (Unity-iPhone_xcodeproj.png).
4. Look for the "Framework Search Path" field (Unity-iPhone_xcodeproj.png).
5. Enter a custom value (e.g. "$(SRCROOT)/Frameworks").
6. Close the Xcode project.
7. Go to the Unity Editor, build an Xcode project on top of the old Xcode project and select 'Append'.
8. Repeat steps 3-4.

Actual behavior:
- "Framework Search Path" field is empty.

Expected behavior:
- "Framework Search Path" field has a previous value.

Reproduced with:
2017.4.18f1, 2019.1.0a13