Fix NPE when dealing with margins collapsing

DEVSIX-8419

Author: vitali-pr

layout/src/main/java/com/itextpdf/layout/margincollapse/MarginsCollapseHandler.java

@@ -33,6 +33,7 @@ This file is part of the iText (R) project.
 import com.itextpdf.layout.renderer.BlockFormattingContextUtil;
 import com.itextpdf.layout.renderer.BlockRenderer;
 import com.itextpdf.layout.renderer.CellRenderer;
+import com.itextpdf.layout.renderer.GridContainerRenderer;
 import com.itextpdf.layout.renderer.IRenderer;
 import com.itextpdf.layout.renderer.LineRenderer;
 import com.itextpdf.layout.renderer.TableRenderer;
@@ -227,7 +228,9 @@ public void endMarginsCollapse(Rectangle layoutBox) {
         }
 
         MarginsCollapse ownCollapseAfter;
-        boolean lastChildMarginJoinedToParent = prevChildMarginInfo != null && prevChildMarginInfo.isIgnoreOwnMarginBottom() && !lastKidCollapsedAfterHasClearanceApplied;
+        final boolean lastChildMarginJoinedToParent = prevChildMarginInfo != null
+                && prevChildMarginInfo.isIgnoreOwnMarginBottom()
+                && !lastKidCollapsedAfterHasClearanceApplied;
         if (lastChildMarginJoinedToParent) {
             ownCollapseAfter = prevChildMarginInfo.getOwnCollapseAfter();
         } else {
@@ -486,7 +489,10 @@ private static boolean lastChildMarginAdjoinedToParent(IRenderer parent) {
     }
 
     private static boolean isBlockElement(IRenderer renderer) {
-        return renderer instanceof BlockRenderer || renderer instanceof TableRenderer;
+        // GridContainerRenderer is inherited from BlockRenderer but only not to copy/paste some overloads.
+        // It doesn't use BlockRenderer#layout internally.
+        return (renderer instanceof BlockRenderer || renderer instanceof TableRenderer)
+                && !(renderer instanceof GridContainerRenderer);
     }
 
     private static boolean hasHeightProp(IRenderer renderer) {

layout/src/test/java/com/itextpdf/layout/element/GridContainerTest.java

@@ -909,4 +909,23 @@ public void autoRepeatPaddingsBordersMarginsTest() throws IOException, Interrupt
         }
         Assert.assertNull(new CompareTool().compareByContent(filename, cmpName, DESTINATION_FOLDER, "diff_"));
     }
+
+    @Test
+    public void marginsCollapsingIssueTest() throws IOException, InterruptedException {
+        String filename = DESTINATION_FOLDER + "marginsCollapsingIssueTest.pdf";
+        String cmpName = SOURCE_FOLDER + "cmp_marginsCollapsingIssueTest.pdf";
+
+        try (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, Boolean.TRUE);
+
+            document.add(div);
+        }
+        Assert.assertNull(new CompareTool().compareByContent(filename, cmpName, DESTINATION_FOLDER, "diff_"));
+    }
 }