Avoid returning null from debugDescribeChildren (#11806)

In some cases, the element tree is not clean but we are required to dump the tree anyway.
To avoid crashing in those cases, we return an explicit null node.

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -845,14 +845,10 @@ abstract class DiagnosticsNode {
 
       for (int i = 0; i < children.length; i++) {
         final DiagnosticsNode child = children[i];
-
+        assert(child != null);
         final TextTreeConfiguration childConfig = _childTextConfiguration(child, config);
         if (i == children.length - 1) {
           final String lastChildPrefixLineOne = '$prefixChildren${childConfig.prefixLastChildLineOne}';
-          if (child == null) {
-            builder.writeRawLine('$lastChildPrefixLineOne<null>');
-            continue;
-          }
           builder.writeRawLine(child.toStringDeep(
             lastChildPrefixLineOne,
             '$prefixChildren${childConfig.childLinkSpace}${childConfig.prefixOtherLines}',
@@ -862,14 +858,8 @@ abstract class DiagnosticsNode {
             builder.writeRaw('$prefixChildren${childConfig.childLinkSpace}${childConfig.footer}');
         } else {
           final TextTreeConfiguration nextChildStyle = _childTextConfiguration(children[i + 1], config);
-
           final String childPrefixLineOne = '$prefixChildren${childConfig.prefixLineOne}';
           final String childPrefixOtherLines ='$prefixChildren${nextChildStyle.linkCharacter}${childConfig.prefixOtherLines}';
-
-          if (child == null) {
-            builder.writeRawLine('$childPrefixLineOne<null>');
-            continue;
-          }
           builder.writeRawLine(child.toStringDeep(childPrefixLineOne, childPrefixOtherLines));
           if (childConfig.footer.isNotEmpty)
             builder.writeRaw('$prefixChildren${nextChildStyle.linkCharacter}${childConfig.footer}');
@@ -2139,13 +2129,18 @@ abstract class DiagnosticableTree extends Diagnosticable {
   /// Returns a list of [DiagnosticsNode] objects describing this node's
   /// children.
   ///
-  /// Children that are offstage should added with `style`
+  /// Children that are offstage should be added with `style` set to
   /// [DiagnosticsTreeStyle.offstage] to indicate that they are offstage.
   ///
+  /// The list must not contain any null entries. If there are explicit null
+  /// children to report, consider [new DiagnosticsNode.message] or
+  /// [DiagnosticsProperty<Object>] as possible [DiagnosticsNode] objects to
+  /// provide.
+  ///
   /// See also:
   ///
   ///  * [RenderTable.debugDescribeChildren], which provides high quality custom
-  ///    descriptions for child nodes.
+  ///    descriptions for its child nodes.
   ///
   /// Used by [toStringDeep], [toDiagnosticsNode] and [toStringShallow].
   @protected

packages/flutter/lib/src/painting/text_span.dart

@@ -353,8 +353,14 @@ class TextSpan extends DiagnosticableTree {
 
   @override
   List<DiagnosticsNode> debugDescribeChildren() {
-    return children == null ?
-        const <DiagnosticsNode>[] :
-        children.map((TextSpan child) => child?.toDiagnosticsNode()).toList();
+    if (children == null)
+      return const <DiagnosticsNode>[];
+    return children.map((TextSpan child) {
+      if (child != null) {
+        return child.toDiagnosticsNode();
+      } else {
+        return new DiagnosticsNode.message('<null child>');
+      }
+    }).toList();
   }
 }

packages/flutter/lib/src/widgets/framework.dart

@@ -3326,7 +3326,11 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   List<DiagnosticsNode> debugDescribeChildren() {
     final List<DiagnosticsNode> children = <DiagnosticsNode>[];
     visitChildren((Element child) {
-      children.add(child.toDiagnosticsNode());
+      if (child != null) {
+        children.add(child.toDiagnosticsNode());
+      } else {
+        children.add(new DiagnosticsNode.message('<null child>'));
+      }
     });
     return children;
   }

packages/flutter/lib/src/widgets/widget_inspector.dart

@@ -101,6 +101,7 @@ class _WidgetInspectorState extends State<WidgetInspector>
     final List<DiagnosticsNode> children = object.debugDescribeChildren();
     for (int i = children.length - 1; i >= 0; i -= 1) {
       final DiagnosticsNode diagnostics = children[i];
+      assert(diagnostics != null);
       if (diagnostics.style == DiagnosticsTreeStyle.offstage ||
           diagnostics.value is! RenderObject)
         continue;

packages/flutter/test/painting/text_span_test.dart

@@ -30,24 +30,24 @@ void main() {
     expect(c1 == b2, isFalse);
   });
 
-  test('TextSpan', () {
+  test('TextSpan toStringDeep', () {
     final TextSpan test = const TextSpan(
       text: 'a',
       style: const TextStyle(
-        fontSize: 10.0
+        fontSize: 10.0,
       ),
       children: const <TextSpan>[
         const TextSpan(
           text: 'b',
           children: const <TextSpan>[
-            const TextSpan()
-          ]
+            const TextSpan(),
+          ],
         ),
         null,
         const TextSpan(
-          text: 'c'
+          text: 'c',
         ),
-      ]
+      ],
     );
     expect(test.toStringDeep(), equals(
       'TextSpan:\n'
@@ -58,7 +58,7 @@ void main() {
       '    "b"\n'
       '    TextSpan:\n'
       '      (empty)\n'
-      '  <null>\n'
+      '  <null child>\n'
       '  TextSpan:\n'
       '    "c"\n'
     ));

packages/flutter/test/widgets/framework_test.dart

@@ -506,4 +506,42 @@ void main() {
       ),
     );
   });
+
+  testWidgets('Element diagnostics with null child', (WidgetTester tester) async {
+    await tester.pumpWidget(new NullChildTest());
+    final NullChildElement test = tester.element<NullChildElement>(find.byType(NullChildTest));
+    test.includeChild = true;
+    expect(
+      tester.binding.renderViewElement.toStringDeep(),
+      equalsIgnoringHashCodes(
+        '[root](renderObject: RenderView#4a0f0)\n'
+        '└NullChildTest(dirty)\n'
+        ' └<null child>\n',
+      ),
+    );
+    test.includeChild = false;
+  });
+}
+
+class NullChildTest extends Widget {
+  @override
+  Element createElement() => new NullChildElement(this);
 }
+
+class NullChildElement extends Element {
+  NullChildElement(Widget widget) : super(widget);
+
+  bool includeChild = false;
+
+  @override
+  void visitChildren(ElementVisitor visitor) {
+    if (includeChild)
+      visitor(null);
+  }
+
+  @override
+  void forgetChild(Element child) { }
+
+  @override
+  void performRebuild() { }
+}
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