On Windows 2008, when setting File Path =
My_Config.txt

It fails with Null pointer exception.

To resolve I use:
${WORKSPACE}/My_Config.txt

Trace is:

Building remotely on AUTOJEN1 (Win2008 Regression IE9) in workspace c:\jenkins-slave\workspace\Run_Test_Suite_debug
[EnvInject] - Executing scripts and injecting environment variables after the SCM step.
java.lang.NullPointerException
at hudson.FilePath.mkdirs(FilePath.java:2891)
at hudson.FilePath.access$800(FilePath.java:190)
at hudson.FilePath$37.invoke(FilePath.java:1904)
at hudson.FilePath$37.invoke(FilePath.java:1901)
at hudson.FilePath.act(FilePath.java:990)
at hudson.FilePath.act(FilePath.java:968)
at hudson.FilePath.write(FilePath.java:1901)
at com.etas.jenkins.plugins.CreateTextFile.CreateFileTask.call(CreateFileTask.java:94)
at com.etas.jenkins.plugins.CreateTextFile.CreateFileTask.call(CreateFileTask.java:38)
at hudson.remoting.UserRequest.perform(UserRequest.java:120)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at hudson.remoting.Engine$1$1.run(Engine.java:62)
at java.lang.Thread.run(Unknown Source)
at ......remote call to AUTOJEN1(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1416)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:220)
at hudson.remoting.Channel.call(Channel.java:781)
at com.etas.jenkins.plugins.CreateTextFile.CreateFileBuilder.perform(CreateFileBuilder.java:82)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:782)
at hudson.model.Build$BuildExecution.build(Build.java:205)
at hudson.model.Build$BuildExecution.doRun(Build.java:162)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:534)
at com.tikal.jenkins.plugins.multijob.MultiJobBuild$MultiJobRunnerImpl.run(MultiJobBuild.java:136)
at hudson.model.Run.execute(Run.java:1738)
at com.tikal.jenkins.plugins.multijob.MultiJobBuild.run(MultiJobBuild.java:73)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Build step 'Create/Update Text File' marked build as failure