Salesforce CPQ error 'Attempt to de-reference null object'

Determine where the 'Attempt to de-reference null object' error is appearing, then follow the steps below to resolve the error.
 

On a New Quote

• Ensure correct the Quote's Pricebook look-up is populated after selection and its Pricebook ID is a valid record ID
• ​If installed package setting 'Solution Groups Enabled' is enabled, ensure the Object and Name features in package settings are populated properly.

When adding a Product Bundle

• If you have a Product-scoped Lookup Rule that is referencing a Lookup Field that does not exist on the Lookup Object

During Quote Configuration

• Check for a valid Optional SKU and Configured SKU product record on all Product Options. Optional SKU's can be data loaded without any product record 
• Attempt to de-reference a null object on Quote Configuration
• Attempt to de-reference a null object on Product Selection
• Attempt to de-reference a null Object in Product Configuration

When a Quote is Ordered

• Confirm the quote field SBQQ__Type__c is not null
• Confirm all Quote Lines have a Quantity populated
•  If a Quote has Quote Lines with MDQ Products, confirm the fields SBQQ__SegmentIndex__c, SBQQ__SegmentKey__c, SBQQ__SegmentLabel__c, SBQQ__StartDate__c, SBQQ__EndDate__c are populated. You can use below query to verify the fields:
   

  SELECT id, SBQQ__Quantity__c, SBQQ__EffectiveQuantity__c, SBQQ__SegmentIndex__c, SBQQ__SegmentKey__c, SBQQ__SegmentLabel__c, SBQQ__StartDate__c, SBQQ__EndDate__c FROM SBQQ__QuoteLine__c WHERE SBQQ__Quote__C = '<<Your Quote Id>>>'

  
  Note: The Quote Id should be inside single quotation marks

Save, Quick Save, Calculate on Quote Line Editor

• List Price or Special Price is populated on all Quote Lines
• Quantity is not null
• A formula (either field or defined in a Price Rule) is referencing a field on an object that is not directly related to the Quote, Quote Line, or Quote Line Group.
• A Quote Line is referencing a RequiredBy Id that does not exist.

During Contract Renewal or Amendment

• Check Subscriptions records' related Product records. Subscription Pricing for these products cannot be null.
• Attempt to de-reference a null object when a contract with legacy data is renewed or amended

When Contracting From Order or Opportunity and setting Contracted to True:

• Confirm if there is an existing Contract tied to the Quote. If so, the quote needs to be deleted or set as null in the SBQQ__Quote__c lookup on the Contract.