Argument Cannot Be Null Error
What does it mean
This error indicates that a method, constructor, or operation was invoked with a null value provided for an argument that is required to be non-null.
The Salesforce platform or the specific code being executed expected a valid object reference or primitive value but received null instead.
Solution
Identify the line of code where the error occurs using debug logs or the error message stack trace.
Examine the variable or expression being passed as an argument at that specific location in your code.
Implement null checks (if (myVariable != null)) before calling the method or using the variable as an argument.
Ensure that variables are properly initialized and assigned values before they are used in operations or passed to methods.
Review the logic that populates the variable to confirm it receives a non-null value when one is mandatory.
Consider using safe navigation operator (?.) or null coalescing (??) in certain contexts if available and appropriate for handling potential nulls gracefully.
Why it happened
The code attempted to perform an action, such as calling a method or accessing a property, on a variable that held a null reference.
Many methods and operations within Apex and the Salesforce platform are explicitly designed to not handle null inputs for critical arguments.
When a null value is supplied where a non-null value is expected or mandatory, the system throws this exception to prevent unexpected behavior, data corruption, or subsequent errors.