Import MS-Project - change - Export

Hey,

I have problems with exporting my MS-Project File.
Thats a section of the error message:
Apr 21, 2017 11:20:36 AM net.sourceforge.ganttproject.GPLogger logToLogger
WARNUNG: Export failure. Failed subtask: Export project
java.lang.RuntimeException: Export failure. Failed subtask: Export project
at net.sourceforge.ganttproject.export.ExporterBase$2.run(ExporterBase.java:167)
at org.bardsoftware.impl.eclipsito.JobManagerImpl$GroupWorker.run(JobManagerImpl.java:163)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at net.sf.mpxj.CustomFieldContainer.registerAlias(CustomFieldContainer.java:109)
at net.sf.mpxj.CustomField.setAlias(CustomField.java:93)
at biz.ganttproject.impex.msproject2.ProjectFileExporter.exportTasks(ProjectFileExporter.java:124)
at biz.ganttproject.impex.msproject2.ProjectFileExporter.run(ProjectFileExporter.java:68)
at biz.ganttproject.impex.msproject2.ExporterToMsProjectFile$2.run(ExporterToMsProjectFile.java:118)
at net.sourceforge.ganttproject.export.ExporterBase$2.run(ExporterBase.java:165)
… 2 more

How can i solve the problem?