MendixIdentifier cannot be cast to java.lang.String after upgrade 7.7.1 to 7.8

I found an issue in a datagrid that contains a calculated field on a reference. But it is not happening for every calculated field.

Does anybody has an idea how to avoid this issue in Mendix 7.8.0, because it was working in 7.7.1 and before?

The corresponding error message is:

java.lang.ClassCastException: com.mendix.basis.objectmanagement.MendixIdentifier cannot be cast to java.lang.String
    at com.mendix.webui.jsonserialization.ResultSerializer$$anonfun$generateMap$1.apply(ResultSerializer.scala:85)
    at com.mendix.webui.jsonserialization.ResultSerializer$$anonfun$generateMap$1.apply(ResultSerializer.scala:85)
    at scala.collection.immutable.Map$Map2.foreach(Map.scala:137)
    at com.mendix.webui.jsonserialization.ResultSerializer.generateMap(ResultSerializer.scala:85)
    at com.mendix.webui.jsonserialization.ResultSerializer.com$mendix$webui$jsonserialization$ResultSerializer$$generate(ResultSerializer.scala:73)
    at com.mendix.webui.jsonserialization.ResultSerializer.generate(ResultSerializer.scala:60)
    at com.mendix.webui.jsonserialization.ResultSerializer$$anonfun$generateMap$1.apply(ResultSerializer.scala:85)
    at com.mendix.webui.jsonserialization.ResultSerializer$$anonfun$generateMap$1.apply(ResultSerializer.scala:85)
    at scala.collection.immutable.HashMap$HashMap1.foreach(HashMap.scala:221)
    at scala.collection.immutable.HashMap$HashTrieMap.foreach(HashMap.scala:428)
    at com.mendix.webui.jsonserialization.ResultSerializer.generateMap(ResultSerializer.scala:85)
    at com.mendix.webui.jsonserialization.ResultSerializer.com$mendix$webui$jsonserialization$ResultSerializer$$generate(ResultSerializer.scala:73)
 

Note: I have created a ticket and test project for this issue and sent it to Mendix.