Returning Null JSON Throwing Type Mismatch Error in VBA

Working with JSON files is much easier (IMHO) if you understand how the JsonConverter processes the JSON into a compound object. Let's look at a simple JSON format (taken from this useful site):

{
  "array": [
    1,
    2,
    3
  ],
  "boolean": true,
  "null": null,
  "number": 123,
  "object": {
    "a": "b",
    "c": "d",
    "e": "f"
  },
  "string": "Hello World"
}

The JsonConverter maps each of these data items into their VBA counterparts.

"array"   maps to Collection   (anytime you see the square brackets [])
"boolean" maps to Boolean
"null"    maps to Null
"number"  maps to Double
"object"  maps to Dictionary   (anytime you see the curly braces {})
"string"  maps to String

So now we can do useful things with your JSON example, such as determine how many entires are in your "issues" array by

Dim issues As Collection
Set issues = schema("issues")
Debug.Print issues.Count

Each of the entries in your "issues" array is actually a compound object itself, so it's a Dictionary. We could, therefore, do something like this:

Dim issue As Variant
For Each issue In issues
    If issue.Exists("id") Then
        Debug.Print "id = " & issue("id")
    End If
Next issue

Of course, the "fields" section of this single issue is itself another Dictionary. So stacking up the dictionary references we can do this too:

Debug.Print "field summary is " & issue("fields")("summary")

All of this is background, hopefully to make it easier on accessing members of a JSON structure. Your real question is on handling NULLs. If the actual value of a field is set to null (see the above sample), then you check it like so

If IsNull(issue("fields")("customfield_13500")) Then ...

A couple of other side notes before we put it all together:

1. Always use Option Explicit
2. Avoid Select and Activate
3. Always define and set references to all Workbooks and Sheets

In the example below, you'll see that I assumed you had to check each field for Null. That is best accomplished by isolating that check in a subroutine rather than over-mess your code with a long string of If statements. The advantage of the code example below is that you don't have to hard-code the number of issues because your logic can detect how many there are.

Option Explicit

Sub main()
    Dim schema As Object
    Set schema = GetJSON("C:\dev\junk.json")

    Dim thisWB As Workbook
    Dim destSH As Worksheet
    Set thisWB = ThisWorkbook
    Set destSH = thisWB.Sheets("Sheet1")

    Dim anchor As Range
    Set anchor = destSH.Range("A1")

    Dim issues As Collection
    Set issues = schema("issues")

    Dim i As Long
    Dim issue As Variant
    For Each issue In issues
        If issue.Exists("id") Then
            SetCell anchor.Cells(1, 1), issue("fields")("issuetype")("name")
            SetCell anchor.Cells(1, 2), issue("key")
            SetCell anchor.Cells(1, 3), issue("fields")("summary")
            '--- if you're not sure if the "name" field is there,
            '    then remember it's a Dictionary so check with Exists
            If issue("fields")("status").Exists("name") Then
                SetCell anchor.Cells(1, 4), issue("fields")("status")("name")
            Else
                SetCell anchor.Cells(1, 4), vbNullString
            End If
            SetCell anchor.Cells(1, 5), issue("fields")("assignee")
            SetCell anchor.Cells(1, 6), issue("fields")("customfield_13301")
            '--- possibly get the Count and iterate over the exact number of components
            For i = 0 To issue("fields")("components").Count - 1
                SetCell anchor.Cells(1, 7), issue("fields")("components")(i)("name")
            Next i
            SetCell anchor.Cells(1, 9), issue("fields")("customfield_13300")
            SetCell anchor.Cells(1, 10), issue("fields")("customfield_10002")
            Set anchor = anchor.Offset(1, 0)
        End If
    Next issue
End Sub

Function GetJSON(ByVal filename As String) As Object
    '--- first ingest the JSON file and get it parsed
    Dim fso As FileSystemObject
    Dim jsonTS As TextStream
    Dim jsonText As String
    Set fso = New FileSystemObject
    Set jsonTS = fso.OpenTextFile(filename, ForReading)
    jsonText = jsonTS.ReadAll
    Set GetJSON = JsonConverter.ParseJson(jsonText)
End Function

Private Sub SetCell(ByRef thisCell As Range, ByVal thisValue As Variant)
    If IsNull(thisValue) Then
        thisCell = vbNullString
    Else
        thisCell = thisValue
    End If
End Sub