DataWeave スクリプトが失敗した場合のトラブルシューティング

You called the function '++' with these arguments:
  1: Null (null)
  2: String (" A text")

But it expects one of these combinations:
  (Array, Array)
  (Date, Time)
  (Date, LocalTime)
  (Date, TimeZone)
  (LocalDateTime, TimeZone)
  (LocalTime, Date)
  (LocalTime, TimeZone)
  (Object, Object)
  (String, String)
  (Time, Date)
  (TimeZone, LocalDateTime)
  (TimeZone, Date)
  (TimeZone, LocalTime)

1| payload.message ++ " A text"
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Trace:
  at ++ (line: 1, column: 1)
  at main (line: 1, column: 17)

You called the function 'Value Selector' with these arguments:

  1: String ("{ \"message\": 123}")
  2: Name ("message")

But it expects one of these combinations:
  (Array, Name)
  (Array, String)
  (Date, Name)
  (DateTime, Name)
  (LocalDateTime, Name)
  (LocalTime, Name)
  (Object, Name)
  (Object, String)
  (Period, Name)
  (Time, Name)

1| payload.message
   ^^^^^^^^^^^^^^^
Trace:
  at main (line: 1, column: 1)

"Trying to output non-whitespace characters outside main element tree (in prolog or epilog), while writing Xml at ." evaluating expression: "payload.order.product.model".

"Expecting type is {
  preamble?: String,
  parts: {
    _*: {
      headers: Object,
      content: Any
    }
  }
} but got Array, while writing MultiPart.
Trace:
  at main (Unknown)" evaluating expression: "dw::core::Objects::keySet(payload.parts)".

"Text plain writer is unable to write Array.
Reason:
Cannot coerce Array (org.mule.weave.v2.model.values.ArrayValue$ArraySeqArrayValue@1331b353) to String
Trace:
  at main (Unknown), while writing TextPlain.
Trace:
  at main (Unknown)" evaluating expression: "payload splitBy ' '".

%dw 2.0
output application/json encoding="sjis"
---
"～―－＄￠￡㈱①"

{
  "Content-Disposition": {
    name: "file",
    filename: "ä\ufffd\ufffd明.txt",
    subtype: "form-data"
  },
  "Content-Type": "text/plain"
}

--myboundary
Content-Disposition: form-data; name="file1"; filename="a.json"
Content-Type: application/json

{
"title": "Java 8 in Action",
"author": "Mario Fusco",
"year": 2014
}
--myboundary
Content-Disposition: form-data; name="file2"; filename="a.xml"
Content-Type: application/xml

<doc>
    <title> Available for download! </title>
    <content> Really large content </content>
</doc>
--myboundary--

%dw 2.0
input payload multipart boundary='myboundary'
output application/json
---
payload.parts mapObject ((value, key, index) ->
{
    (key): {
	fileName: payload.parts[index].headers.'Content-Disposition'.filename,
    rawContent: payload.parts[index].content.^raw
    }
})