Classic mode web application in SharePoint 2013 Error occurs

while creating Classic web applicatiuon using powershell commands following error occure

PS C:\Users\centraluser> New-SPWebApplication -Name "SharePoint - 100" -Applicat
ionPool "SharePoint100" -AuthenticationMethod "NTLM" -ApplicationPoolAccount (Ge
t-SPManagedAccount "wse\centraluser") -Port 100 -URL "http://w8-sp13-en"
WARNING: The Windows Classic authentication method is deprecated in this
release and the default behavior of this cmdlet, which creates Windows Classic
based web application, is obsolete. It is recommended to use Claims
authentication methods. You can create a web application that uses Claims
authentication method by specifying the AuthenticationProvider parameter set in
 this cmdlet. Refer to the http://go.microsoft.com/fwlink/?LinkId=234549 site
for more information. Please note that the default behavior of this cmdlet is
expected to change in the future release to create a Claims authentication
based web application instead of a Windows Classic based web application.
log4net:ERROR XmlConfigurator: ConfigureFromXml called with null 'element' param
eter
log4net:ERROR XmlConfigurator: Failed to find configuration section 'log4net' in
 the application's .config file. Check your .config file for the <log4net> and <
configSections> elements. The configuration section should look like: <section n
ame="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net" /
>
New-SPWebApplication : Object reference not set to an instance of an object.
At line:1 char:1
+ New-SPWebApplication -Name "SharePoint - 100" -ApplicationPool
"SharePoint100" - ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~
    + CategoryInfo          : InvalidData: (Microsoft.Share...PWebApplication:
   SPCmdletNewSPWebApplication) [New-SPWebApplication], NullReferenceExceptio
  n
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletNewSPWeb
   Application

PS C:\Users\centraluser>