how to solve this ??Attempt to de-reference a null object

Try below VF page

<apex:page standardcontroller="Cross_Sell_Opportunity__c"  extensions="CrossSellRExtensionController">
 <apex:sectionHeader title="Title" subtitle="Person Accounts"/>
    <apex:form >
		<apex:outputPanel >
		</apex:outputPanel>
    <apex:pageMessages />
    <apex:pageBlock title="Please fill in the details for creating *Contacts*">
		<apex:pageBlockSection >
			<apex:inputField value="{!Cross_Sell_Opportunity__c.Name}" required="true"/><br/>
			<apex:inputField value="{!newPersonAccount.Salutation}" /> &nbsp;
            <apex:inputField value="{!newPersonAccount.FirstName}"  /><br/>
            <apex:inputField value="{!newPersonAccount.LastName}"  required="true"/><br/>
            <apex:inputField value="{!newPersonAccount.PersonEmail}" required="true"/><br/>
            <apex:inputField value="{!newPersonAccount.Status__c}" /><br/>
        </apex:pageBlockSection>
        <apex:pageBlockButtons >
            <apex:commandButton action="{!save}" value="Save" /> 
            <apex:commandButton action="{!cancel}" value="Cancel"/>
		</apex:pageBlockButtons>
	</apex:pageBlock>
    </apex:form>
</apex:page>

Below controller class

public class CrossSellRExtensionController {
    public Account newPersonAccount {get; set;}
	public CrossSellRExtensionController(ApexPages.StandardController controller) 
	{
           Cross_Sell_Opportunity__c csr = (Cross_Sell_Opportunity__c) controller.getRecord();
           csr = [Select Id, Name  from Cross_Sell_Opportunity__c where Id =: csr.Id];
           RecordType personAccountRecordType =  [SELECT Id FROM RecordType WHERE Name = 'Person Account' and SObjectType = 'Account'];
           newPersonAccount = new Account();
           newPersonAccount.Cross_Sell_Referral__c = csr.Id;
		   newPersonAccount.recordtypeId = personAccountRecordType.id ;
    }
	public PageReference save() 
	{
			insert newPersonAccount; //this is line 17
			PageReference pageRef = new PageReference('/' + newPersonAccount.Cross_Sell_Referral__c);
			pageRef.setRedirect(true);
			ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'new *contact* created.'));
			return pageRef;
	}
	public PageReference cancel() 
	{
		 return new PageReference('/'+ newPersonAccount.Cross_Sell_Referral__c);
	}
}

Let us know if this will help you