Fixed parameter element editor

Now allows 'Add Folder' to append on already present elements

com.unity.perception/CHANGELOG.md

 
 Fixed ground truth not properly produced when there are other disabled PerceptionCameras present. Note: this does not yet add support for multiple enabled PerceptionCameras.
 
+Fixed an issue preventing a user from adding more parameters to a parameter list with the 'Add Folder' button. 'Add Folder' now appends the contents of the new folder on the list instead of replacing its contents.
+
 ## [0.7.0-preview.2] - 2021-02-08
 
 ### Upgrade Notes

com.unity.perception/Editor/Randomization/VisualElements/Parameter/ParameterElement.cs

                     if (folderPath == string.Empty)
                         return;
                     var categories = LoadAssetsFromFolder(folderPath, categoricalParameter.sampleType);
+
+                    var probabilityIndex = probabilitiesProperty.arraySize;
+
+                    var optionsIndex = optionsProperty.arraySize;
+
-                        var optionProperty = optionsProperty.GetArrayElementAtIndex(i);
-                        var probabilityProperty = probabilitiesProperty.GetArrayElementAtIndex(i);
+                        var optionProperty = optionsProperty.GetArrayElementAtIndex(optionsIndex + i);
+                        var probabilityProperty = probabilitiesProperty.GetArrayElementAtIndex(probabilityIndex + i);
                         optionProperty.objectReferenceValue = categories[i];
                         probabilityProperty.floatValue = uniformProbability;
                     }