better exception message in visitors

source/RobotAPI/libraries/aron/core/data/visitor/RecursiveVisitor.h

@@ -57,7 +57,17 @@ namespace armarx::aron::data
         virtual void visitDouble(Input& element) {};
         virtual void visitBool(Input& element) {};
         virtual void visitString(Input& element) {};
-        virtual void visitUnknown(Input& element) { throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor."); }
+        virtual void visitUnknown(Input& element)
+        {
+            if (!element)
+            {
+                throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor. The element was NULL.");
+            }
+            else
+            {
+                throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor. The element was set but descriptor was undefined.");
+            }
+        }
         virtual ~RecursiveVisitor() = default;
     };
 
@@ -108,8 +118,16 @@ namespace armarx::aron::data
         virtual void visitBool(DataInput& elementData, TypeInput& elementType) {};
         virtual void visitString(DataInput& elementData, TypeInput& elementType) {};
         virtual void visitAnyObject(DataInput& elementData, TypeInput& elementType) {};
-        virtual void visitUnknown(DataInput& elementData, TypeInput& elementType) {
-            throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor.");
+        virtual void visitUnknown(DataInput& elementData, TypeInput& elementType)
+        {
+            if (!elementData)
+            {
+                throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor. The element was NULL.");
+            }
+            else
+            {
+                throw error::AronException(__PRETTY_FUNCTION__, "Unknown type in visitor. The element was set but descriptor was undefined.");
+            }
         }
         virtual ~RecursiveTypedVisitor() = default;
     };