Begin of the insertionIf no null value exists when grouping by a list custom field, the front end runs into an error:

application.js:1141 TypeError: Failed to execute 'appendChild' on 'Node': parameter 1 is not of type 'Node'.
at http://localhost:3000/assets/application.js:6506:26
at Array.forEach (native)
at GroupedRowsBuilder.internalBuildRows (http://localhost:3000/assets/application.js:6497:20)
at GroupedRowsBuilder.RowsBuilder.buildRows (http://localhost:3000/assets/application.js:6856:21)
at WorkPackageTable.refreshBody (http://localhost:3000/assets/application.js:11666:39)
at WorkPackageTable.initialSetup (http://localhost:3000/assets/application.js:11660:14)
at SafeSubscriber._next (http://localhost:3000/assets/application.js:11445:19)
at SafeSubscriber.__tryOrUnsub (http://localhost:3000/assets/application.js:1982:16)
at SafeSubscriber.next (http://localhost:3000/assets/application.js:1931:22)
at Subscriber._next (http://localhost:3000/assets/application.js:1884:26)

This is caused by incompatible urls between the hrefs in the groups, e.g.:

```
"groups": [
{
"value": "896",
"count": 1,
"_links": {
"valueLink": [
{
"href": "/api/v3/string_objects?value=896"
}
],
"groupBy": {
"href": "/api/v3/queries/group_bys/customField385",
"title": "Custom field 385 en"
}
}
},
{
"value": "898",
"count": 3,
"_links": {
"valueLink": [
{
"href": "/api/v3/string_objects?value=898"
}
],
"groupBy": {
"href": "/api/v3/queries/group_bys/customField385",
"title": "Custom field 385 en"
}
}
},
{
"value": "899",
"count": 1,
"_links": {
"valueLink": [
{
"href": "/api/v3/string_objects?value=899"
}
],
"groupBy": {
"href": "/api/v3/queries/group_bys/customField385",
"title": "Custom field 385 en"
}
}
}
]
```

compared to the url in the value of a work package that is to be grouped:

```
"customField385": {
"title": "CF 385 de value 4",
"href": "/api/v3/string_objects?value=896&title=CF%20385%20de%20value%204"
}
```End of the insertion