When an orange exclamation mark appears it is a warning message. The "cannot process null" message is informing you that there is a field you are trying to evaluate but the value is returning null in which case it cannot evaluate against a null value. You can either safely ignore this message or exclude any null values in your search results and this will prevent the warning message from appearing.
You can use the where operator to exclude any null values for a specific field like shown as below:
| where !isNull(<field_name>)