I recently encounter this error. It took a little bit of head scratching to work out why we were getting the error. The possible causes were quite high as we are using dynamically assigned maps (see code below).
Message Assignment Shape Code
||SapOrderMapType = System.Type.GetType(SapOrdersMapName);
//transform(MyOutputMsg) = MyMapType(MyInputMsg);
transform (SapOrder) = SapOrderMapType(CanonicalPO);
System.Diagnostics.Trace.WriteLineIf(TraceEnabled, "Transform to SAP schema complete.", TraceCategory);
SapOrder(FILE.ReceivedFileName) = "PO_" + CustomerNumber + "_" + PONumber + "_";
System.Diagnostics.Trace.WriteLineIf(TraceEnabled, "SAP Order Msg Created", TraceCategory);
The error information identifies the transform as the problem but not what the problem is. The inner exception is no additional help- ‘Value cannot be null. Parameter name: extension Exception type: ArgumentNullException’.
The problem was eventually tracked down to the custom functoids. In the last map a new functoid was added to convert standard BizTalk dates into standard SAP dates. When the project was deployed the custom functoid dll was not updated.
Interestingly this error might occur intermittently as the functiod may not be needed every time the map is used.