Memory Layers disappear from project if one is made permanent.

[mikedufty]

We have been having issues with memory layers sometimes losing all features when reopening saved projects.
I have now found a way to reproduce this.
It appears to be triggered by using the save scratch layer function (clicking on memory chip icon to right of layer in layer pane).
Tested by
1 create blank project
2 add two temporary layers and add features to them.
3 use the save scratch layer function to convert one to a shape file.
4 save, close and reopen the project, the other scratch layer has lost it's features. Display memory layer information shows 0 features.

Using 3.28.14 LTR and up to date memory layer plugin.

In other projects which have older memory layers imported from previous versions of the plugin, it appears the older memory layers may retain features.

[mikedufty]

Test.zip
Added example files
MLtest1.qgz has 3 memory layers.
MLtest2.qgz is the same file saved after 'save scratch layer' function used to convert the lake layer to shapefile.

[YoannQDQ]

Great work, thanks. I don't have much time to work on that at the moment, but hopefully I'll be able to fix it by the end of the month

[YoannQDQ]

Finaly had some time to fix this. Version 5.0.2 is waiting for approval.
#15 might be a duplicate of this issue, but I'm waiting for feedback on 5.0.2 before closing it.