Fixes #6318: Made widgets more error-tolerant and user-friendly when certain option values are null

Collapse.php

@@ -94,7 +94,7 @@ class Collapse extends Widget
         $items = [];
         $index = 0;
         foreach ($this->items as $item) {
-            if (!isset($item['label'])) {
+            if (!array_key_exists('label', $item)) {
                 throw new InvalidConfigException("The 'label' option is required.");
             }
             $header = $item['label'];
@@ -116,7 +116,7 @@ class Collapse extends Widget
      */
     public function renderItem($header, $item, $index)
     {
-        if (isset($item['content'])) {
+        if (array_key_exists('content', $item)) {
             $id = $this->options['id'] . '-collapse' . $index;
             $options = ArrayHelper::getValue($item, 'contentOptions', []);
             $options['id'] = $id;

Dropdown.php

@@ -84,7 +84,7 @@ class Dropdown extends Widget
                 $lines[] = $item;
                 continue;
             }
-            if (!isset($item['label'])) {
+            if (!array_key_exists('label', $item)) {
                 throw new InvalidConfigException("The 'label' option is required.");
             }
             $encodeLabel = isset($item['encode']) ? $item['encode'] : $this->encodeLabels;

Tabs.php

@@ -142,7 +142,7 @@ class Tabs extends Widget
         }
 
         foreach ($this->items as $n => $item) {
-            if (!isset($item['label'])) {
+            if (!array_key_exists('label', $item)) {
                 throw new InvalidConfigException("The 'label' option is required.");
             }
             $encodeLabel = isset($item['encode']) ? $item['encode'] : $this->encodeLabels;
@@ -216,7 +216,7 @@ class Tabs extends Widget
             if (is_string($item)) {
                 continue;
             }
-            if (!isset($item['content'])) {
+            if (!array_key_exists('content', $item)) {
                 throw new InvalidConfigException("The 'content' option is required.");
             }