Stack trace:
Caused by: java.lang.NullPointerException
at org.drools.container.spring.namespace.KnowledgeSessionDefinitionParser.parseInternal(KnowledgeSessionDefinitionParser.java:321)
at org.springframework.beans.factory.xml.AbstractBeanDefinitionParser.parse(AbstractBeanDefinitionParser.java:59)
at org.springframework.beans.factory.xml.NamespaceHandlerSupport.parse(NamespaceHandlerSupport.java:73)
at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1335)
at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1325)
at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.parseBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:135)
at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.registerBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:93)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.registerBeanDefinitions(XmlBeanDefinitionReader.java:493)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(XmlBeanDefinitionReader.java:390)

For example, my applicationContext.xml:

.....
<bean id="config" factory-method="getConfiguration" factory-bean="configFactory" />
.....

solution:

class: org.drools.container.spring.namespace.KnowledgeSessionDefinitionParser
method: parseInternal

line :
if ( def.getBeanClassName().equals( KnowledgeAgentBeanFactory.class.getName() ) ) {

should be replaced with somethig like this:
if ( KnowledgeAgentBeanFactory.class.getName().equals( def.getBeanClassName() ) ) {