TypeError: Cannot read property 'validateName' of undefined

I'm new in ReactJs. In my application, I have inserted the following code to validate textArea, but when I type on it, it throws following error.

NewBrand.js:67 Uncaught TypeError: Cannot read property 'validateName' of undefined
        at handleNameChange (NewBrand.js:67)
        at TextArea._this.handleTextareaChange (TextArea.js:94)
        at HTMLUnknownElement.callCallback (react-dom.development.js:100)
        at Object.invokeGuardedCallbackDev (react-dom.development.js:138)
        at Object.invokeGuardedCallback (react-dom.development.js:187)
        at Object.invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:201)
        at executeDispatch (react-dom.development.js:461)
        at executeDispatchesInOrder (react-dom.development.js:483)
        at executeDispatchesAndRelease (react-dom.development.js:581)
        at executeDispatchesAndReleaseTopLevel (react-dom.development.js:592)

My code looks like follows:

    handleNameChange(event) {
        const value = event.target.value;
        this.setState({
        name: {
            text: value,
            ...this.validateName(value)
        }
    });

    }

    isFormInvalid() {
        if(this.state.name.validateStatus !== 'success') {
            return true;
        }
    }

  validateName = (nameText) => {
            if(nameText.length === 0) {
                return {
                    validateStatus: 'error',
                    errorMsg: 'Please enter the Brand name!'
                }
            } else if (nameText.length > POLL_QUESTION_MAX_LENGTH) {
                return {
                    validateStatus: 'error',
                    errorMsg: `Name is too long (Maximum ${POLL_QUESTION_MAX_LENGTH} characters allowed)`
                }
            } else {
                return {
                    validateStatus: 'success',
                    errorMsg: null
                }
            }
        }

Please hepl me to resolve this issue