A field Descriptor was created by mixing the message Descriptor from one proto message with the field number from another. For example:

Foo.getDescriptor().findFieldByNumber(Bar.ID_FIELD_NUMBER)

This accesses the Descriptor of a field in Foo with a field number from Bar. One of these was probably intended:

Foo.getDescriptor().findFieldByNumber(Foo.ID_FIELD_NUMBER)
Bar.getDescriptor().findFieldByNumber(Bar.ID_FIELD_NUMBER)