Merge branch 'cherry-pick-9caf02cb' into 'releases/2.6'

Merge branch 'fix/father-initialization' into 'master'

See merge request !40

dune/foamgrid/foamgrid/foamgridelements.hh

@@ -63,7 +63,9 @@ namespace Dune {
               vertex_{{nullptr, nullptr}},
               facet_{{nullptr, nullptr}},
               sons_{{nullptr, nullptr}},
-              nSons_(0), refinementIndex_(-1),
+              nSons_(0),
+              father_(nullptr),
+              refinementIndex_(-1),
               markState_(DO_NOTHING), isNew_(false),
               coarseningBlocked_(false),
               growthInsertionIndex_(-1)