Fix ExpansionPanelList Duplicate Global Keys Exception (#31228)

- Move `setState` to only be invoked when guarded by `widget._allowMultiplePanelsOpen`, fixing the case for `ExpansionPanelList`

- Remove setting `_currentOpenPanel` to `widget.initialOpenPanelValue` in `didUpdateWidget`, since this should only occur on `initState` and not every time the widget is updated. This fixes the problem for `ExpansionPanelList.radio`

- Added a `didUpdateWidget` condition for when `ExpansionPanelList` changes into `ExpansionPanelList.radio` to open the panel at `widget.initialOpenPanelValue`

- Added test cases for regression, expansionCallback cases, and `didUpdateWidget` transitions between `ExpansionPanelList` and `ExpansionPanelList.radio`

packages/flutter/lib/src/material/expansion_panel.dart

@@ -133,6 +133,9 @@ class ExpansionPanelRadio extends ExpansionPanel {
 /// A material expansion panel list that lays out its children and animates
 /// expansions.
 ///
+/// Note that [expansionCallback] behaves differently for [ExpansionPanelList]
+/// and [ExpansionPanelList.radio].
+///
 /// {@tool snippet --template=stateful_widget_scaffold}
 ///
 /// Here is a simple example of how to implement ExpansionPanelList.
@@ -319,8 +322,17 @@ class ExpansionPanelList extends StatefulWidget {
 
   /// The callback that gets called whenever one of the expand/collapse buttons
   /// is pressed. The arguments passed to the callback are the index of the
-  /// to-be-expanded panel in the list and whether the panel is currently
-  /// expanded or not.
+  /// pressed panel and whether the panel is currently expanded or not.
+  ///
+  /// If ExpansionPanelList.radio is used, the callback may be called a
+  /// second time if a different panel was previously open. The arguments
+  /// passed to the second callback are the index of the panel that will close
+  /// and false, marking that it will be closed.
+  ///
+  /// For ExpansionPanelList, the callback needs to setState when it's notified
+  /// about the closing/opening panel. On the other hand, the callback for
+  /// ExpansionPanelList.radio is simply meant to inform the parent widget of
+  /// changes, as the radio panels' open/close states are managed internally.
   ///
   /// This callback is useful in order to keep track of the expanded/collapsed
   /// panels in a parent widget that may need to react to these changes.
@@ -348,11 +360,9 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
   void initState() {
     super.initState();
     if (widget._allowOnlyOnePanelOpen) {
-      assert(_allIdentifiersUnique(), 'All object identifiers are not unique!');
-      for (ExpansionPanelRadio child in widget.children) {
-        if (widget.initialOpenPanelValue != null &&
-            child.value == widget.initialOpenPanelValue)
-          _currentOpenPanel = child;
+      assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
+      if (widget.initialOpenPanelValue != null) {
+        _currentOpenPanel = searchPanelByValue(widget.children, widget.initialOpenPanelValue);
       }
     }
   }
@@ -360,14 +370,15 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
   @override
   void didUpdateWidget(ExpansionPanelList oldWidget) {
     super.didUpdateWidget(oldWidget);
+
     if (widget._allowOnlyOnePanelOpen) {
-      assert(_allIdentifiersUnique(), 'All object identifiers are not unique!');
-      for (ExpansionPanelRadio newChild in widget.children) {
-        if (widget.initialOpenPanelValue != null &&
-            newChild.value == widget.initialOpenPanelValue)
-          _currentOpenPanel = newChild;
+      assert(_allIdentifiersUnique(), 'All ExpansionPanelRadio identifier values must be unique.');
+      // If the previous widget was non-radio ExpansionPanelList, initialize the
+      // open panel to widget.initialOpenPanelValue
+      if (!oldWidget._allowOnlyOnePanelOpen) {
+        _currentOpenPanel = searchPanelByValue(widget.children, widget.initialOpenPanelValue);
       }
-    } else if (oldWidget._allowOnlyOnePanelOpen) {
+    } else {
       _currentOpenPanel = null;
     }
   }
@@ -395,6 +406,8 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
     if (widget._allowOnlyOnePanelOpen) {
       final ExpansionPanelRadio pressedChild = widget.children[index];
 
+      // If another ExpansionPanelRadio was already open, apply its
+      // expansionCallback (if any) to false, because it's closing.
       for (int childIndex = 0; childIndex < widget.children.length; childIndex += 1) {
         final ExpansionPanelRadio child = widget.children[childIndex];
         if (widget.expansionCallback != null &&
@@ -402,9 +415,19 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
             child.value == _currentOpenPanel?.value)
           widget.expansionCallback(childIndex, false);
       }
-      _currentOpenPanel = isExpanded ? null : pressedChild;
+
+      setState(() {
+        _currentOpenPanel = isExpanded ? null : pressedChild;
+      });
+    }
+  }
+
+  ExpansionPanelRadio searchPanelByValue(List<ExpansionPanelRadio> panels, Object value)  {
+    for (ExpansionPanelRadio panel in panels) {
+      if (panel.value == value)
+        return panel;
     }
-    setState(() { });
+    return null;
   }
 
   @override