Steps to reproduce:
1. Download 2019.2.0f1.zip
2. Create a shader graph and add Custom Function
3. Press the Gear button
4. Open Source input fields selection window
5. Select any script (do not close object selection window)
6. Select another script

Expected results: Whit new script selection Source input field is updated in the shader graph
Actual results: Just the first-time Source field is updated.

Reproduced with: 2019.3.0a12, 2019.2.1f1

Reproduced with Shader graph version: 7.0.1, 6.9.1, 6.9.0 (in older versions of shader graph Custom Function node does not exist)

Note:
- With different nodes and Object input field, this issue isn't reproducible in Shader Graph