flex-sdk_mustella-air tests are failing

[Tom Chiverton]

Twice in a row now these tests have failed with the following stack.
I don't think we've changed these tests, but could it be set to use a 
directory that doesn't exist or something ?

TypeError: Error #1009: Cannot access a property or method of a null 
object reference.
    at mx.controls.fileSystemClasses::FileSystemControlHelper/set 
directory()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\frameworks\projects\airframework\src\mx\controls\fileSystemClasses\FileSystemControlHelper.as:242]
    at mx.controls::FileSystemTree/set 
directory()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\frameworks\projects\airframework\src\mx\controls\FileSystemTree.as:292]
    at 
FileTree_properties_tester/setDir()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\tests\apollo\FileTree\properties\FileTree_properties_tester.mxml:66]
    at 
FileTree_properties_tester/___FileTree_properties_tester_RunCode3_code()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\tests\apollo\FileTree\properties\FileTree_properties_tester.mxml:139]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at 
RunCode/doStep()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\RunCode.as:46]
    at 
TestStep/execute()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\TestStep.as:65]
    at 
TestCase/runSteps()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\TestCase.as:418]
    at 
TestCase/runSetup()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\TestCase.as:272]
    at 
TestCase/runNextSteps()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\TestCase.as:463]
    at 
UnitTester$/enterFrameHandler()[C:\jenkins_slave\workspace\flex-sdk_mustella-air\mustella\as3\src\mustella\UnitTester.as:1175]


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*Tom Chiverton*
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