`Mapper` was changed to `Properties` but are there...

# serialization

[Robert Jaros]

Mapper was changed to Properties but are there any other internal changes in this class? I get NPE when trying to use it the same way as in 1.3.61 ?

[Robert Jaros]

I call 'loadNullable' and I get NPE - it makes little sense for me ;-)

[Robert Jaros]

There is something wrong with checking "size" property in the map

[Robert Jaros]

https://github.com/Kotlin/kotlinx.serialization/issues/743