Fix invalid feedback visibility of checkboxes and radios for Bootstrap 4

E.g. with Laravel:
@php
    $validator = Validator::make(request()->all(), [
        'failed_checkbox' => 'required',
        'failed_radio' => 'required',
        'failed_inline_checkbox' => 'required',
        'failed_inline_radio' => 'required',
        'failed_checkboxes' => 'required',
        'failed_inline_checkboxes' => 'required',
    ]);

    $validator->fails();
    Former::withErrors($validator);
@endphp

{!! Former::open()->method('GET') !!}
    {!! Former::checkbox('failed_checkbox')->text('Failed checkbox') !!}
    {!! Former::radio('failed_radio')->text('Failed radio') !!}
{!! Former::checkbox('failed_inline_checkbox')->text('Failed inline
checkbox')->inline() !!}
{!! Former::radio('failed_inline_radio')->text('Failed inline
radio')->inline() !!}
{!! Former::checkboxes('failed_checkboxes')->checkboxes('first',
'second', 'third', 'fourth') !!}
{!!
Former::checkboxes('failed_inline_checkboxes')->checkboxes('first',
'second', 'third', 'fourth')->inline() !!}
{!! Former::close() !!}

Reference:
formers#581 (comment)

src/Former/Form/Group.php

@@ -175,8 +175,17 @@ public function contents($contents)
 	public function wrapField($field)
 	{
 		$label = $this->getLabel($field);
+		$help = $this->getHelp();
+		if ($field->isCheckable() && $this->app['former']->framework() == 'TwitterBootstrap4') {
+			$wrapperClass = $field->isInline() ? 'form-check form-check-inline' : 'form-check';
+			if ($this->app['former']->getErrors($field->getName())) {
+				$hiddenInput = Element::create('input', null, ['type' => 'hidden'])->class('form-check-input is-invalid');
+				$help = $hiddenInput.$help;
+			}
+			$help = Element::create('div', $help)->class($wrapperClass);
+		}
 		$field = $this->prependAppend($field);
-		$field .= $this->getHelp();
+		$field .= $help;
 
 		return $this->wrap($field, $label);
 	}

src/Former/Traits/Checkable.php

@@ -257,6 +257,17 @@ public function check($checked = true)
 		return $this;
 	}
 
+
+	/**
+	 * Check if the checkables are inline
+	 *
+	 * @return boolean
+	 */
+	public function isInline()
+	{
+		return $this->inline;
+	}
+
 	////////////////////////////////////////////////////////////////////
 	////////////////////////// INTERNAL METHODS ////////////////////////
 	////////////////////////////////////////////////////////////////////
@@ -391,7 +402,6 @@ protected function createCheckable($item, $fallbackValue = 1)
 			$wrapper_class = $this->inline ? 'form-check form-check-inline' : 'form-check';
 
 			$element = Element::create('div', $element)->class($wrapper_class)->render();
-
 		} else {
 			// Original way is to add the 'input' inside the 'label'
 			$element = Element::create('label', $field.$label)->for($attributes['id'])->class($class)->render();