description:
exception was detected when selecting binding in value/category series if chart sharing result from other report item

build number: 
2.5.2.v20100210-0630

steps to reproduce:
1, new a table binding with data set
2, new a chart share the result set from table or insert the chart into table header
3, go to select data page in chart builder
4, select binding from binding list of value y or category x series

expected result:
no exception is thrown in backgroud

actual result:
check the error log, npe thrown in background

error log:
java.lang.nullpointerexception
	at org.eclipse.birt.chart.ui.swt.wizard.data.basedatadefinitioncomponent.gettooltipfordatatext(basedatadefinitioncomponent.java:591)
	at org.eclipse.birt.chart.ui.swt.wizard.data.basedatadefinitioncomponent.settooltipforinputcontrol(basedatadefinitioncomponent.java:570)
	at org.eclipse.birt.chart.ui.swt.wizard.data.basedatadefinitioncomponent.onmodifyexpression(basedatadefinitioncomponent.java:528)
	at org.eclipse.birt.chart.ui.swt.wizard.data.basedatadefinitioncomponent$2.handleevent(basedatadefinitioncomponent.java:334)
	at org.eclipse.birt.chart.reportitem.ui.chartexpressionbuttonutil$chartexpressionbutton.notifychangeevent(chartexpressionbuttonutil.java:301)
	at org.eclipse.birt.chart.reportitem.ui.chartexpressionbuttonutil$chartexpressionbutton.onchange(chartexpressionbuttonutil.java:281)
	at org.eclipse.birt.chart.reportitem.ui.chartexpressionbuttonutil$chartexpressionbutton.access$0(chartexpressionbuttonutil.java:277)
	at org.eclipse.birt.chart.reportitem.ui.chartexpressionbuttonutil$chartexpressionbutton$controllistener.handleevent(chartexpressionbuttonutil.java:240)
	at org.eclipse.swt.widgets.eventtable.sendevent(eventtable.java:84)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1003)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1027)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1012)
	at org.eclipse.swt.widgets.widget.notifylisteners(widget.java:770)
	at org.eclipse.swt.custom.ccombo.handlefocus(ccombo.java:844)
	at org.eclipse.swt.custom.ccombo$3.handleevent(ccombo.java:141)
	at org.eclipse.swt.widgets.eventtable.sendevent(eventtable.java:84)
	at org.eclipse.swt.widgets.display.filterevent(display.java:1208)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1002)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1027)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1008)
	at org.eclipse.swt.widgets.control.sendfocusevent(control.java:2583)
	at org.eclipse.swt.widgets.widget.wmsetfocus(widget.java:2282)
	at org.eclipse.swt.widgets.control.wm_setfocus(control.java:4573)
	at org.eclipse.swt.widgets.button.wm_setfocus(button.java:1216)
	at org.eclipse.swt.widgets.control.windowproc(control.java:4014)
	at org.eclipse.swt.widgets.display.windowproc(display.java:4619)
	at org.eclipse.swt.internal.win32.os.callwindowprocw(native method)
	at org.eclipse.swt.internal.win32.os.callwindowproc(os.java:2314)
	at org.eclipse.swt.widgets.button.callwindowproc(button.java:338)
	at org.eclipse.swt.widgets.widget.wmlbuttondown(widget.java:1853)
	at org.eclipse.swt.widgets.control.wm_lbuttondown(control.java:4297)
	at org.eclipse.swt.widgets.button.wm_lbuttondown(button.java:1197)
	at org.eclipse.swt.widgets.control.windowproc(control.java:3981)
	at org.eclipse.swt.widgets.display.windowproc(display.java:4632)
	at org.eclipse.swt.internal.win32.os.dispatchmessagew(native method)
	at org.eclipse.swt.internal.win32.os.dispatchmessage(os.java:2411)
	at org.eclipse.swt.widgets.display.readanddispatch(display.java:3501)
	at org.eclipse.jface.window.window.runeventloop(window.java:825)
	at org.eclipse.jface.window.window.open(window.java:801)
	at org.eclipse.birt.core.ui.frameworks.taskwizard.wizardbase.open(wizardbase.java:87)
	at org.eclipse.birt.chart.ui.swt.wizard.chartwizard.open(chartwizard.java:234)
	at org.eclipse.birt.chart.reportitem.ui.chartreportitembuilderimpl.open(chartreportitembuilderimpl.java:287)
	at org.eclipse.birt.report.designer.internal.ui.processor.extenedelementprocessor.editelement(extenedelementprocessor.java:64)
	at org.eclipse.birt.report.designer.internal.ui.views.defaultnodeprovider.performinsert(defaultnodeprovider.java:729)
	at org.eclipse.birt.report.designer.internal.ui.views.defaultnodeprovider.performrequest(defaultnodeprovider.java:410)
	at sun.reflect.nativemethodaccessorimpl.invoke0(native method)
	at sun.reflect.nativemethodaccessorimpl.invoke(unknown source)
	at sun.reflect.delegatingmethodaccessorimpl.invoke(unknown source)
	at java.lang.reflect.method.invoke(unknown source)
	at org.eclipse.birt.report.designer.ui.views.elementadapterinvocationhandler.invoke(elementadapterinvocationhandler.java:89)
	at $proxy0.performrequest(unknown source)
	at org.eclipse.birt.report.designer.ui.actions.baseinsertmenuaction.insertelement(baseinsertmenuaction.java:193)
	at org.eclipse.birt.report.designer.ui.actions.baseinsertmenuaction.run(baseinsertmenuaction.java:271)
	at org.eclipse.jface.action.action.runwithevent(action.java:498)
	at org.eclipse.jface.action.actioncontributionitem.handlewidgetselection(actioncontributionitem.java:584)
	at org.eclipse.jface.action.actioncontributionitem.access$2(actioncontributionitem.java:501)
	at org.eclipse.jface.action.actioncontributionitem$5.handleevent(actioncontributionitem.java:411)
	at org.eclipse.swt.widgets.eventtable.sendevent(eventtable.java:84)
	at org.eclipse.swt.widgets.widget.sendevent(widget.java:1003)
	at org.eclipse.swt.widgets.display.rundeferredevents(display.java:3910)
	at org.eclipse.swt.widgets.display.readanddispatch(display.java:3503)
	at org.eclipse.ui.internal.workbench.runeventloop(workbench.java:2405)
	at org.eclipse.ui.internal.workbench.runui(workbench.java:2369)
	at org.eclipse.ui.internal.workbench.access$4(workbench.java:2221)
	at org.eclipse.ui.internal.workbench$5.run(workbench.java:500)
	at org.eclipse.core.databinding.observable.realm.runwithdefault(realm.java:332)
	at org.eclipse.ui.internal.workbench.createandrunworkbench(workbench.java:493)
	at org.eclipse.ui.platformui.createandrunworkbench(platformui.java:149)
	at org.eclipse.ui.internal.ide.application.ideapplication.start(ideapplication.java:113)
	at org.eclipse.equinox.internal.app.eclipseapphandle.run(eclipseapphandle.java:194)
	at org.eclipse.core.runtime.internal.adaptor.eclipseapplauncher.runapplication(eclipseapplauncher.java:110)
	at org.eclipse.core.runtime.internal.adaptor.eclipseapplauncher.start(eclipseapplauncher.java:79)
	at org.eclipse.core.runtime.adaptor.eclipsestarter.run(eclipsestarter.java:368)
	at org.eclipse.core.runtime.adaptor.eclipsestarter.run(eclipsestarter.java:179)
	at sun.reflect.nativemethodaccessorimpl.invoke0(native method)
	at sun.reflect.nativemethodaccessorimpl.invoke(unknown source)
	at sun.reflect.delegatingmethodaccessorimpl.invoke(unknown source)
	at java.lang.reflect.method.invoke(unknown source)
	at org.eclipse.equinox.launcher.main.invokeframework(main.java:559)
	at org.eclipse.equinox.launcher.main.basicrun(main.java:514)
	at org.eclipse.equinox.launcher.main.run(main.java:1311)
