Erro ao Gerar JavaDoc no NetBeans...Alguem já viu esse erro?

init:
Warning: Leaving out empty argument '-windowtitle'
Generating Javadoc
Javadoc execution
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\ArquivoObjetos.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\Interface.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Celular.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\CelularAssinatura.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\CelularCartao.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\CelularInexistenteException.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Cliente.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\DataHoraErradaException.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Endereco.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\GeradorNumero.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Ligacoes.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Operadora.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\Plano.java...
Loading source file C:\Users\Julio\Documents\NetBeansProjects\celular\src\celular\SaldoInsuficienteException.java...
Constructing Javadoc information...
Standard Doclet version 1.6.0_24
Building tree for all the packages and classes...
java.lang.NullPointerException
        at com.sun.tools.doclets.formats.html.PackageUseWriter.generatePackageUse(PackageUseWriter.java:180)
        at com.sun.tools.doclets.formats.html.PackageUseWriter.generatePackageList(PackageUseWriter.java:124)
        at com.sun.tools.doclets.formats.html.PackageUseWriter.generatePackageUse(PackageUseWriter.java:110)
        at com.sun.tools.doclets.formats.html.PackageUseWriter.generatePackageUseFile(PackageUseWriter.java:99)
        at com.sun.tools.doclets.formats.html.PackageUseWriter.generate(PackageUseWriter.java:78)
        at com.sun.tools.doclets.formats.html.ClassUseWriter.generate(ClassUseWriter.java:116)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.generateOtherFiles(HtmlDoclet.java:92)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:122)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractDoclet.java:64)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:42)
        at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:269)
        at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:143)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
        at com.sun.tools.javadoc.Start.begin(Start.java:128)
        at com.sun.tools.javadoc.Main.execute(Main.java:41)
        at com.sun.tools.javadoc.Main.main(Main.java:31)
C:\Users\Julio\Documents\NetBeansProjects\celular\nbproject\build-impl.xml:695: Javadoc returned 1
FALHA NA CONSTRUÇÃO (tempo total: 2 segundos)