Internal server error while running in replay mode

org.eclipse.swt.SWTException: Failed to execute runnable (java.lang.IllegalStateException: Not a JSON Object: null)
org.eclipse.swt.SWT.error(SWT.java:3531)
org.eclipse.swt.SWT.error(SWT.java:3454)
org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:185)
org.eclipse.swt.widgets.Display.runPendingMessages(Display.java:1148)
org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:1135)
org.eclipse.rwt.internal.lifecycle.ProcessAction.execute(ProcessAction.java:27)
org.eclipse.rwt.internal.lifecycle.PhaseExecutor.execute(PhaseExecutor.java:35)
org.eclipse.rwt.internal.lifecycle.SimpleLifeCycle.execute(SimpleLifeCycle.java:133)
org.eclipse.rwt.internal.service.LifeCycleServiceHandler.runLifeCycle(LifeCycleServiceHandler.java:99)
org.eclipse.rwt.internal.service.LifeCycleServiceHandler.handlePostRequest(LifeCycleServiceHandler.java:92)
org.eclipse.rwt.internal.service.LifeCycleServiceHandler.synchronizedService(LifeCycleServiceHandler.java:67)
org.eclipse.rwt.internal.service.LifeCycleServiceHandler.service(LifeCycleServiceHandler.java:58)
org.eclipse.rwt.engine.RWTServlet.handleValidRequest(RWTServlet.java:68)
org.eclipse.rwt.engine.RWTServlet.doPost(RWTServlet.java:47)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.eclipse.rap.rwt.osgi.internal.CutOffContextPathWrapper.service(CutOffContextPathWrapper.java:106)
org.eclipse.equinox.http.servlet.internal.ServletRegistration.service(ServletRegistration.java:61)
org.eclipse.equinox.http.servlet.internal.ProxyServlet.processAlias(ProxyServlet.java:128)
org.eclipse.equinox.http.servlet.internal.ProxyServlet.service(ProxyServlet.java:60)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.eclipse.equinox.servletbridge.BridgeServlet.service(BridgeServlet.java:120)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)