Merged column headers should work within declarative Grid. (#12206)

- null check to prevent NPE from the merged column headers
- convert to internal columnIds for merge handling
- use correct cell in merge handling
- switch away from streams in merge handling for easier readability
- regression test

Fixes: #10464

2 files changed, 15 insertions, 10 deletions

diff --git a/server/src/main/java/com/vaadin/ui/Grid.java b/server/src/main/java/com/vaadin/ui/Grid.java
index 5d7534ae6a..dbde93b558 100644
--- a/server/src/main/java/com/vaadin/ui/Grid.java
+++ b/server/src/main/java/com/vaadin/ui/Grid.java
@@ -4762,7 +4762,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
             for (Column<T, ?> c : getColumns()) {
                 HeaderCell headerCell = getDefaultHeaderRow().getCell(c);
                 if (headerCell.getCellType() == GridStaticCellType.TEXT) {
-                    c.setCaption(headerCell.getText());
+                    String text = headerCell.getText();
+                    c.setCaption(text == null ? "" : text);
                 }
             }
         }
diff --git a/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java b/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
index 29af8bfc6c..96e374253a 100644
--- a/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
+++ b/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
@@ -290,18 +290,22 @@ public abstract class StaticSection<ROW extends StaticSection.StaticRow<?>>
                                     + columnIdsString + "'");
                 }
 
-                Stream.of(columnIds).forEach(this::addCell);
-
-                Stream<String> idsStream = Stream.of(columnIds);
+                CELL cell;
                 if (colspan > 1) {
-                    CELL newCell = createCell();
-                    addMergedCell(createCell(),
-                            idsStream.collect(Collectors.toSet()));
-                    newCell.readDesign(element, designContext);
+                    Set<String> columnGroup = new HashSet<>();
+                    for (String columnId : columnIds) {
+                        addCell(columnId);
+                        // convert the public columnIds into internal columnIds
+                        columnGroup.add(getCell(columnId).getColumnId());
+                    }
+                    cell = createCell();
+                    addMergedCell(cell, columnGroup);
                 } else {
-                    idsStream.map(this::getCell).forEach(
-                            cell -> cell.readDesign(element, designContext));
+                    String columnId = columnIds[0];
+                    addCell(columnId);
+                    cell = getCell(columnId);
                 }
+                cell.readDesign(element, designContext);
             }
         }