fix: SCHWED-4345 allow returning null pointer.

This allows IsValid() to work as intended without throwing an exception.
Because the validity of the underlying entity is now checked in CheckValidity,
it should no longer be a problem.

modules/mol/base/src/impl/atom_impl.cc

@@ -73,9 +73,6 @@ void AtomImpl::Apply(EntityVisitor& v)
 
 ResidueImplPtr AtomImpl::GetResidue() const
 {
-  if (!res_.lock()) {
-    throw InvalidHandle();
-  }
   return res_.lock();
 }
 
@@ -218,7 +215,7 @@ AtomImpl::~AtomImpl() {
 EntityImplPtr AtomImpl::GetEntity() const
 {
   if (!res_.lock()) {
-    throw InvalidHandle();
+    return EntityImplPtr();
   }
   return res_.lock()->GetEntity();
 }

modules/mol/base/src/impl/chain_impl.cc

@@ -369,9 +369,6 @@ AtomImplPtr ChainImpl::FindAtom(const ResNum& num,
 
 EntityImplPtr ChainImpl::GetEntity() const 
 {
-  if (!ent_.lock()) {
-    throw InvalidHandle();
-  }
   return ent_.lock();
 }
 int ChainImpl::GetIndexForResNum(const ResNum& number) const

modules/mol/base/src/impl/residue_impl.cc

@@ -200,9 +200,6 @@ void ResidueImpl::SetSecStructure(SecStructure ss)
 
 EntityImplPtr ResidueImpl::GetEntity() const
 {
-  if (!ent_.lock()) {
-    throw InvalidHandle();
-  }
   return ent_.lock();
 }
 
@@ -259,9 +256,6 @@ int ResidueImpl::GetBondCount() const
 
 ChainImplPtr ResidueImpl::GetChain() const
 {
-  if (!chain_.lock()) {
-    throw InvalidHandle();
-  }
   return chain_.lock();
 }