Help with Power BI Template and Odata null values

Hi folks,

I have a problem I'm hoping the community can help with.

I have a Power BI template file for connecting to Azure DevOps data (using OData as source). With the template users enter their organization and project name, hit load and have charts auto populated for them. They can also select any custom fields they have added, like so:

So the whole query reads like so:
https://analytics.dev.azure.com/{Organization}/{Project}/_odata/v3.0/WorkItems?%20$filter=WorkItemType%20ne%20%27Task%27%20and%20WorkItemType%20ne%20%27Test%20Case%27%20and%20WorkItemType%20ne%20%27Test%20Plan%27%20and%20WorkItemType%20ne%20%27Shared%20Parameter%27%20and%20WorkItemType%20ne%20%27Shared%20Steps%27%20and%20WorkItemType%20ne%20%27Test%20Suite%27%20and%20State%20ne%20%27Removed%27%20&$select=AreaSK,CompletedDate,CompletedDateSK,CycleTimeDays,IterationSK,LeadTimeDays,ResolvedDate,State,StateCategory,Title,ValueArea,WorkItemID,WorkItemType,{Custom Fields}

Problem is, if they don't have any custom fields I want them to be able to leave the field blank and the report still load, however when they do that I see the following message:

The full source in Power Query reads like so:
= OData.Feed("https://analytics.dev.azure.com/" & Organization & "/" & Project & "/_odata/v3.0/WorkItems?%20$filter=WorkItemType%20ne%20%27Task%27%20and%20WorkItemType%20ne%20%27Test%20Case%27%20and%20WorkItemType%20ne%20%27Test%20Plan%27%20and%20WorkItemType%20ne%20%27Shared%20Parameter%27%20and%20WorkItemType%20ne%20%27Shared%20Steps%27%20and%20WorkItemType%20ne%20%27Test%20Suite%27%20and%20State%20ne%20%27Removed%27%20&$select=AreaSK,CompletedDate,CompletedDateSK,CycleTimeDays,IterationSK,LeadTimeDays,ResolvedDate,State,StateCategory,Title,ValueArea,WorkItemID,WorkItemType," & #"Custom Fields", null, [Implementation="2.0"])

Any tips for what I should be doing differently?