Expression.Error: We cannot convert the value null to type Text. Details: Value= Type=Type

I managed to work this out. I first thought the null error was being thrown inside the custom function fnGetAssetVulnerabilityDetails, so I tried to sanitise nulls before concatenating the string, but that didn't work. I then thought the null error was being thrown by the function header, so I changed the function declaration from

let fnGetAssetVulnerabilityDetails = (assetId as text, pluginId as text) =>

to

let fnGetAssetVulnerabilityDetails = (assetId as any, pluginId as any) =>

to allow nulls to come in, but that didn't work either.

I then discovered the error was being thrown in my main procedure while calling the function. I don't understand why Power BI wasn't passing null parameters, but added a step to remove the nulls before calling the function.

    #"Change Plugin ID data type" = Table.TransformColumnTypes(#"Expand vulnerabilities",{{"Plugin ID", type text}}),
    #"Replace null Plugin IDs" = Table.TransformColumns(#"Change Plugin ID data type", {"Plugin ID", each if _ is null then "0" else _}),
    #"Get vulnerability details" = Table.AddColumn(#"Replace null Plugin IDs", "VulnerabilityDetails", each fnGetAssetVulnerabilityDetails([Tenable Asset UUID], [Plugin ID])),

I hope this helps someone else.

I managed to figure this out. I first thought that the null error was being thrown inside the fnGetAssetVulnerabilityDetailscustom function, so I tried to disinfect the nulls before concatenating the string, but that didn't work. So I thought the function header threw a null error, so I changed the declaration of the function

let fnGetAssetVulnerabilityDetails = (assetId as text, pluginId as text) =>

a

let fnGetAssetVulnerabilityDetails = (assetId as any, pluginId as any) =>

to allow nulls, but it didn't work either.

Then I discovered that the error was being generated in my main procedure when calling the function. I don't understand why Power BI wasn't passing null parameters, but added a step to remove nulls before calling the function.

    #"Change Plugin ID data type" = Table.TransformColumnTypes(#"Expand vulnerabilities",{{"Plugin ID", type text}}),
    #"Replace null Plugin IDs" = Table.TransformColumns(#"Change Plugin ID data type", {"Plugin ID", each if _ is null then "0" else _}),
    #"Get vulnerability details" = Table.AddColumn(#"Replace null Plugin IDs", "VulnerabilityDetails", each fnGetAssetVulnerabilityDetails([Tenable Asset UUID], [Plugin ID])),

I hope this helps someone else.