Either you have failed deployed application or deleted an autodeployed application from the autodeploy folder when server was stopped.

Possible Cause:
Server subsystem failed. A NULL pointer exception: java.lang.NullPointerException 

Stop the Weblogic app server and remove the <app-deployment> tag entry of the failed/deleted application  from the Weblogic config.xml file. After making required changes, Start the server again. If it does not work, kill the process have lock on any of the deployed file or restart/reboot the system to release the lock.

Note: Please make sure to have a backup of config.xml file, before making any changes.

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