Requirement : based on employee id need to display assets(like laptop ,mobile,etc) issued to him by employer
created user__C lookup(user) field on asset

<apex:page standardController="Asset" extensions="assetNewExtension">
<apex:form >
<apex:outputPanel id="userDetails">
<apex:pageBlock >
<apex:pageBlockSection columns="1">
<apex:inputField value="{!Asset.User__c}">
<apex:actionSupport event="onchange" action="{!readUser}" reRender="userDetails"/>
</apex:inputField>
<apex:outputField value="{!asset.Quantity}"/>
<apex:outputField value="{!asset.Quantity}"/>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:outputPanel>
</apex:form>
</apex:page>

public with sharing class assetNewExtension { public User selectedUser { get; set; }
public Asset asset { get; set; }
public assetNewExtension(ApexPages.StandardController controller) { }
public assetNewExtension()
  {
Asset = new asset();
Asset.User__c = UserInfo.getUserId();
selectedUser = [ Select Id From User Where Id = :UserInfo.getUserId() ];
  }
public pageReference readUser() {
Asset = [ Select Quantity From asset Where Id =:Asset.User__c];
return null;
      }
}

after selecting user from look up i got thss error

System.NullPointerException: Attempt to de-reference a null object
Error is in expression '{!readUser}' in page vfpage: Class.assetNewExtension.readUser: line 17, column 1
Class.assetNewExtension.readUser: line 17, column 1