could not find LOCAL DTD/Schema with publicId [null] and the file/resource is [simple-methods.xsd]

[nachi19-2]

hi all,
         i am working on pos for making the refund of a particular item in the order,  
i am getting this warning message  which is making the application to perform slow,can anyone tell me why it is giving such a warning message and how to resolve it.    
     [UtilXml.LocalResolver.resolveEntity] could not find LOCAL DTD/Schema with publicId [null] and the file/resource is [simple-methods.xsd]