Emulated multi-function calls within one request

API
16

I’ve been stuck for awhile as well in implementing emulated function calls as I’m planning to create an AI tutor that assists me in the English language. There are instances that it works well but there are also times that it outputs this error “InvalidRequestError: Invalid value for ‘content’: expected a string, got null.” Let me know if there are incorrect approaches done in my function call and I am open to suggestions to improve the performance of my personal project.

{
    "name": "error_checker_function",
    "description":"Call three functions in one call",
    "parameters":{
        "type":"object",
        "properties":{
            "check_grammatical_error":{
                "name":"check_grammatical_error",
                "description": "Check for grammatical errors from the input",
                "parameters":{
                    "type":"object",
                    "properties":{
                        "grammar_error":{
                            "type":"string",
                            "description":"Parts of a text that do not follow standard English grammar rules, e.g I is happy"
                        },
                        "grammar_correction":{
                            "type":"string",
                            "description":"Correct the grammatical error that violates English grammar rules from the input, e.g. Instead of saying 'I is happy' it should be 'I'm happy' or 'I am happy'"
                        }
                    },
                    "required": ["grammar_error", "grammar_correction"],
                }
            },
            "check_spelling_error":{
                "name":"check_spelling_error",
                "description": "Check for spelling errors from the input",
                "parameters":{
                    "type":"object",
                    "properties":{
                        "spelling_error":{
                            "type":"string",
                            "description":"Mistakes made in writing when the proper letter arrangement in a word is not followed in the English language, e.g 'Riecive'; 'Acomodate'"
                        },
                        "spelling_correction":{
                            "type":"string",
                            "description":"Correct the improper letter arrangement done from the input, e.g. 'Receive' is the proper spelling and not 'Riecive'; 'Accomodate' is the correct spelling, not 'Acomodate'"
                        }
                    },
                    "required": ["spelling_error", "spelling_correction"],
                }
            },
            "check_improper_punctuations":{
                "name":"check_improper_punctuations",
                "description": "Check for imporer punctuations usage from the input",
                "parameters":{
                    "type":"object",
                    "properties":{
                        "punctuation_error":{
                            "type":"string",
                            "description":"Incorrect punctuation usage that does not follow proper English punctuation rules, e.g The cat's are playful"
                        },
                        "punctuation_correction":{
                            "type":"string",
                            "description":"Correct the improper usage of punctuations from the input, e.g. Instead of saying 'The cat's are playful' it should be 'The cats are playful'"
                        }
                    },
                    "required": ["punctuation_error", "punctuation_correction"],
                }
            }
        }, "required":["check_grammatical_error","check_spelling_error","check_improper_punctuations"]
    }
}