Project compatibility is lost too easily

Hello.

I am struggling against a strange problem that project compatibilty is
volatile and lost even without any code change.

To reproduce:
    1. In ProjectBuilder, save and close project.
            (the project can be opened until quitting squeak)
    2. quit squeak without saving
    3. restart squeak
    4. open the saved project, that results "EToysWorld has changed
class version" error.

How can I workaround (or fix) it?
Thanks in advance.

/Korakurider (a tweak learner)

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