When did the process run last? Please review the steps or rules you have written since then. You are passing a null string value. When you find the location of this add the "" in the field or cell that is empty. I cannot be more specific without knowing what you were working on.
Looks like you may have included a period "." in a string literal and Pega thinks you are trying to reference a Property value using dot "." notation somewhere in your process. Double check all the Audit Notes used in all the Flow Shapes in your Flow Rules, also double check all Assignment Shape Instructions (also in your Flow Rules) as well as the Subject field for any Correspondence Activities you may be using. Finally, double check all string literals you are using in any Declare Expressions or Decision Rules you may have defined. If you want to include any punctuation in any string literal you must include the string literal in double quotes. For example: "Hello World." (Notice the period within the string literal is included in double quotes.)