BUG: Create CellData when requested in Mesh::GetCellData

Similar to ::GetPointData(), create a new container object for the cell
data when requested with ::GetCellData(), which prevents client use of
nullptr's.

Author: thewtex

Modules/Core/Mesh/include/itkMesh.hxx

@@ -145,6 +145,10 @@ template <typename TPixelType, unsigned int VDimension, typename TMeshTraits>
 auto
 Mesh<TPixelType, VDimension, TMeshTraits>::GetCellData() -> CellDataContainer *
 {
+  if (!m_CellDataContainer)
+  {
+    this->SetCellData(CellDataContainer::New());
+  }
   itkDebugMacro("returning CellData container of " << m_CellDataContainer);
   return m_CellDataContainer;
 }

Modules/Core/Mesh/test/itkMeshCellDataTest.cxx

@@ -16,9 +16,10 @@
  *
  *=========================================================================*/
 
-#include <itkDefaultDynamicMeshTraits.h>
-#include <itkMesh.h>
-#include <itkTriangleCell.h>
+#include "itkDefaultDynamicMeshTraits.h"
+#include "itkMesh.h"
+#include "itkTriangleCell.h"
+#include "itkTestingMacros.h"
 
 int
 itkMeshCellDataTest(int, char *[])
@@ -44,6 +45,8 @@ itkMeshCellDataTest(int, char *[])
   mesh->SetPoint(4, TPoint{ { { 4.0, 0.0 } } });
   mesh->SetPoint(5, TPoint{ { { 5.0, 1.0 } } });
 
+  ITK_TEST_EXPECT_TRUE(mesh->GetCellData() != nullptr);
+
   {
     TCell::CellAutoPointer cellpointer;
     cellpointer.TakeOwnership(new TTriangle);