It's probably near to work, but the m variable contains :
[{\"id\":null,\"children\":[{\"id\":null,\"children\":[{\"id\":null,\"children\":[]},{\"id\":null,\"children\":[{\"id\":null,\"children\":[{\"id\":null,\"children\":[]}]}]}]},{\"id\":null,\"children\":[]},{\"id\":null,\"children\":[]},{\"id\":null,\"children\":[]},{\"id\":null,\"children\":[]},{\"id\":null,\"children\":[]}]}]
It seems like the serialize functions found the childrens, but is unable to get their id and their contents.
Never mind !
I was stupid enough to forget the id property in the JSON source file ! If I don't provide the id property to Mif tree, it can't does anything else to indicates "null" !
Now, it works perfectly !
Many thanks for that tool ! :-)