"Tool button action "null::null" is an invalid callable" when using @export_tool_button

### Tested versions

Reproducible in v4.4.dev3.official [f4af8201b], v4.4.dev6.official [1f47e4c4e]. These are the only ones i tested.

### System information

Godot v4.4.dev6 - Windows 10.0.22631 - Multi-window, 2 monitors - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2060 SUPER (NVIDIA; 32.0.15.6590) - Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz (8 threads)

### Issue description

(I would leave a video but github didn't accept it)

I made a `@export_tool_button` in in of my scripts. It worked perfectly fine. Then i made another scene and tried to use it there. Gave me two issues.
Issue one was `The value of property "insertVarHere" is Nil, but Callable was expected.`(#97834), and the issue i am making now which is `Tool button action "null::null" is an invalid callable`.

In the Steps to reproduce section i mention that reloading the project worked, but that only worked in the MRP not in my main project.

### Steps to reproduce

To get the error `Tool button action "null::null" is an invalid callable` you need to comment out the `= funky` and restart the project.
Then uncomment it and press the button.

To fix it reload the project while everything is uncommented.

### Minimal reproduction project (MRP)

[toolbuttonbug.zip](https://github.com/user-attachments/files/18202509/toolbuttonbug.zip)

Start with the attached toolbuttonbug.zip minimal reproduction and reproduce the sequence around @export_tool_button: comment out = funky, restart, uncomment it, and press the button. Compare this with reloading the project while uncommented. Done means the button no longer reports "null::null" as an invalid callable in the reproduced sequence.

Written by the indexing model from the issue text.