Panic when attempting to add an invalid FormItem

CHANGELOG

@@ -9,6 +9,7 @@ v1.5.0 (WIP)
 - Revamp FormItem styling as FormItem.SetAttributes
 - Provide DropDownOption in DropDown handlers
 - Provide ListItem in List handlers
+- Panic when attempting to add an invalid FormItem
 
 v1.4.9 (2020-09-08)
 - Add InputField.GetCursorPosition and InputField.SetCursorPosition

form.go

@@ -1,6 +1,7 @@
 package cview
 
 import (
+	"reflect"
 	"sync"
 
 	"github.com/gdamore/tcell/v2"
@@ -454,6 +455,10 @@ func (f *Form) AddFormItem(item FormItem) *Form {
 	f.Lock()
 	defer f.Unlock()
 
+	if reflect.ValueOf(item).IsNil() {
+		panic("Invalid FormItem")
+	}
+
 	f.items = append(f.items, item)
 	return f
 }