Fix NPE when dealing with margins collapsing

DEVSIX-8419

Autoported commit.
Original commit hash: [b35c28b99]

Author: vitali-pr

itext.tests/itext.layout.tests/itext/layout/element/GridContainerTest.cs

@@ -874,5 +874,20 @@ public virtual void AutoRepeatPaddingsBordersMarginsTest() {
             NUnit.Framework.Assert.IsNull(new CompareTool().CompareByContent(filename, cmpName, DESTINATION_FOLDER, "diff_"
                 ));
         }
+
+        [NUnit.Framework.Test]
+        public virtual void MarginsCollapsingIssueTest() {
+            String filename = DESTINATION_FOLDER + "marginsCollapsingIssueTest.pdf";
+            String cmpName = SOURCE_FOLDER + "cmp_marginsCollapsingIssueTest.pdf";
+            using (Document document = new Document(new PdfDocument(new PdfWriter(filename)))) {
+                Div grid = new GridContainer().SetBackgroundColor(ColorConstants.BLUE);
+                grid.Add(new Paragraph("some grid text"));
+                Div div = new Div().SetBackgroundColor(ColorConstants.RED).Add(new Paragraph("some div text")).Add(grid);
+                div.SetProperty(Property.COLLAPSING_MARGINS, true);
+                document.Add(div);
+            }
+            NUnit.Framework.Assert.IsNull(new CompareTool().CompareByContent(filename, cmpName, DESTINATION_FOLDER, "diff_"
+                ));
+        }
     }
 }

itext.tests/itext.layout.tests/resources/itext/layout/GridContainerTest/cmp_marginsCollapsingIssueTest.pdf

@@ -455,7 +455,9 @@ private static bool LastChildMarginAdjoinedToParent(IRenderer parent) {
         }
 
         private static bool IsBlockElement(IRenderer renderer) {
-            return renderer is BlockRenderer || renderer is TableRenderer;
+            // GridContainerRenderer is inherited from BlockRenderer but only not to copy/paste some overloads.
+            // It doesn't use BlockRenderer#layout internally.
+            return (renderer is BlockRenderer || renderer is TableRenderer) && !(renderer is GridContainerRenderer);
         }
 
         private static bool HasHeightProp(IRenderer renderer) {

itext/itext.layout/itext/layout/margincollapse/MarginsCollapseHandler.cs

@@ -1 +1 @@
-ca150c317a7bf138f31ac93c5e2e8542aa7ba789
+b35c28b993564c2a7eb0be14fe05d3e5a323f125