webtop not starting in Tomcat

I have a customized webtop 6.7SP1 application. After upgrading the application to 6.7 SP2 and deploying it on a new tomcat 7 server, the application does not start. How do I fix this issue and start up the application?

The error displayed is:

SEVERE: Exception starting filter WDKController
java.lang.NullPointerException
    at java.util.regex.Pattern.<init>(Unknown Source)
    at java.util.regex.Pattern.compile(Unknown Source)
    at com.documentum.web.env.WDKController.init(Unknown Source)
    at org.apache.catalina.core.ApplicationFilterConfig.initFilter(ApplicationFilterConfig.java:277)
    at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:258)
    at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:382)
    at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:103)
    at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4650)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5306)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:618)
    at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1100)
    at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1618)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Upgrading WDK applications is not straightforward process, you cannot just transfer your modifications and expect them to run.

I doubt that you used recommended approach for upgrading your WDK application. Check this link.

Best approach is to deploy OOTB Webtop 6.7 SP2 and add custom layer, if necessary one by one feature (something which is pretty easy if you used adequate techniques of saving source in source version control systems).

In Documentum System 6.7 SP2 Upgrade and Migration Guide you have section Migrating WDK and Webtop Applications pages 81-125. Read it carefully and review your upgrade actions.

For files you need to migrate take care of these files: (from WDK deployment guide)