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

Hi community

I have a Desktop report that calls a web API provided by Tenable, a security vulnerability scanner, to retrieve a list of IT assets and associated vulnerabilities. The report calls one endpoint to get a list of assets, then three other endpoints to append columns for the identified vulnerabilities for each asset. The preview works correctly, but the following error is thrown when applying changes:

Failed to save modifications to the server. Error returned: "OLE DB or ODBC error: [DataSource.Error] Web.Contents failed to get contents from [URL] (500): Internal Server Error."

I used the method descriped by Reza Rad (2018) to create two refrences to my original query, one removing error rows and one keeping error rows:

let
    Source = #"Tenable - Original",
    #"Removed Errors" = Table.RemoveRowsWithErrors(Source)
in
    #"Removed Errors"

However, this doesn't prevent the error from occuring, so this method must not move all errors. When I look at my reference where I have kept the error rows, I see the following error:

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

I worked backwards, and identified the error is being thrown by the #"Expand vulnerability details" step in my query:

let
    #"Get JSON object" = fnGetJsonFromTenable("https://cloud.tenable.com/workbenches/assets?date_range=30&filter.0.quality=eq&filter.0.filter=has_plugin_results&filter.0.value=true"),
    #"Drill down list" = #"Get JSON object"[assets],
    #"Convert to table" = Table.FromList(#"Drill down list", Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Expand column" = Table.ExpandRecordColumn(#"Convert to table", "Column1",
        {"ipv4", "fqdn", "netbios_name", "operating_system", "has_agent", "id", "last_seen"},
        {"IPv4", "DNS", "NetBIOS", "OS", "Tenable Agent Installed", "Tenable Asset UUID", "Asset Last Seen"}
    ),
    #"Expand IPv4" = Table.ExpandListColumn(#"Expand column", "IPv4"),
    #"Expand DNS" = Table.ExpandListColumn(#"Expand IPv4", "DNS"),
    #"Expand NetBIOS" = Table.ExpandListColumn(#"Expand DNS", "NetBIOS"),
    #"Expand OS" = Table.ExpandListColumn(#"Expand NetBIOS", "OS"),
    #"Get asset location" = Table.AddColumn(#"Expand OS", "Location", each fnGetAssetTagByName([Tenable Asset UUID], "Location")),
    #"Expand location" = Table.ExpandTableColumn(#"Get asset location", "Location", {"Tag Value"}, {"Location"}),
    #"Get asset vulnerabilities" = Table.AddColumn(#"Expand location", "Vulnerabilities", each fnGetAssetVulnerabilities([Tenable Asset UUID])),
    #"Expand vulnerabilities" = Table.ExpandTableColumn(#"Get asset vulnerabilities", "Vulnerabilities", {"Plugin ID", "Plugin Name", "Plugin Family"}),
    #"Change Plugin ID to text" = Table.TransformColumnTypes(#"Expand vulnerabilities",{{"Plugin ID", type text}}),
    #"Get vulnerability details" = Table.AddColumn(#"Change Plugin ID to text", "VulnerabilityDetails", each fnGetAssetVulnerabilityDetails([Tenable Asset UUID], [Plugin ID])),
    #"Expand vulnerability details" = Table.ExpandTableColumn(#"Get vulnerability details", "VulnerabilityDetails", {"Vulnerability First Seen", "Vulnerability Last Seen", "Patch Published", "CVSSv3 Base Score", "CVSSv3 Temporal Score", "CPE", "Exploitable", "Exploit Ease"}),
    #"Change data types" = Table.TransformColumnTypes(#"Expand vulnerability details",{{"IPv4", type text}, {"DNS", type text}, {"NetBIOS", type text}, {"OS", type text}, {"Tenable Agent Installed", type logical}, {"Tenable Asset UUID", type text}, {"Asset Last Seen", type datetime}, {"Location", type text}, {"Plugin Name", type text}, {"Plugin Family", type text}, {"Vulnerability First Seen", type datetime}, {"Vulnerability Last Seen", type datetime}, {"Patch Published", type datetime}, {"CVSSv3 Base Score", type number}, {"CVSSv3 Temporal Score", type number}, {"CPE", type text}, {"Exploitable", type logical}, {"Exploit Ease", type text}})

in
    #"Change data types"

Because the preview works correctly, I don't know which row is causing the error, and therefore how to resolve it. I believe the fnGetAssetVulnerabilityDetails custom function called in step #"Get vulnerability details" isn't always returning a valid structure, but I don't know how to fix that. The function is defined as:

let
    fnGetAssetVulnerabilityDetails = (assetId as text, pluginId as text) =>
    let
        JsonObject = fnGetJsonFromTenable("https://cloud.tenable.com/workbenches/assets/" & assetId & "/vulnerabilities/" & pluginId & "/info"),
        ConvertToTable = Record.ToTable(JsonObject),
        DeleteNameColumn = Table.RemoveColumns(ConvertToTable, {"Name"}),
        ExpandValue = Table.ExpandRecordColumn(DeleteNameColumn, "Value",
            {"discovery", "plugin_details", "risk_information", "vulnerability_information"},
            {"discovery", "plugin_details", "risk_information", "vulnerability_information"}
        ),
        ExpandDiscovery = Table.ExpandRecordColumn(ExpandValue, "discovery",
            {"seen_first", "seen_last"},
            {"Vulnerability First Seen", "Vulnerability Last Seen"}
        ),
        ExpandPluginDetails = Table.ExpandRecordColumn(ExpandDiscovery, "plugin_details",
            {"publication_date"},
            {"Patch Published"}
        ),
        ExpandRiskInformation = Table.ExpandRecordColumn(ExpandPluginDetails, "risk_information",
            {"cvss3_base_score", "cvss3_temporal_score"},
            {"CVSSv3 Base Score", "CVSSv3 Temporal Score"}
        ),
        ExpandVulnerabilityInformation = Table.ExpandRecordColumn(ExpandRiskInformation, "vulnerability_information",
            {"cpe", "exploit_available", "exploitability_ease"},
            {"CPE", "Exploitable", "Exploit Ease"}
        ),
        ReplaceNullCpe = Table.TransformColumns(ExpandVulnerabilityInformation, {"CPE", each if _ is null then {""} else _}),
        ExtractCpe = Table.TransformColumns(ReplaceNullCpe, {"CPE", each Text.Combine(List.Transform(_, Text.From), ","), type text})
    in
        ExtractCpe 
in
    fnGetAssetVulnerabilityDetails

For context. the following table provides sample data from the report: 

I'd be grateful for any suggestions or advice.