Here are the explanations for the three different types of warnings you’re seeing.
Reference to non-existent field 'IsAverageImageMeanPerChannel'.
Reference to non-existent field 'Cropping'.
Dot indexing is not supported for variables of this type.
---> As for the dot indexing not being supported, this is a known behaviour. There was a change in the representation of Network Layers from R2018b to R2019a. Models created in R2018b should work fine in R2019a but the other way around will have issues.