fil.js

const File = () => {
  return (
  <FormItem label="Year Built" error={this.state.yearError}>
    <NumberInput 
      min={0}
      onChange={(yearBuilt) => { 
            // *********** clear your error message here
            this.setState({ yearError: null })  
    
            if (yearBuilt > currentYear || isNaN(yearBuilt)) { // checking for isNaN here may not make sense, since it's a number input it'll always be a number or null
              return (this.setState({ yearError: 'Fix year' })); // probably could use a more descriptive error like year built can't be greater than this year
            }
    
            if (this.state.yearError==null){ // I typically use !this.state.yearError rather than this.state.yearError==null.. also you want to use === triple equals rather than double equals
              yearBuilt = yearBuilt.toString() // if year is null, you can't call toString().... toString() is a method on a number... also you should call this once during on submit rather than every single onChange
            }
            return this.setState({ yearBuilt })}
     
        }} 
     />
</FormItem>
  )
}