use operation stack when disconnecting

app/node/output/viewer/viewer.cpp

@@ -52,7 +52,10 @@ ViewerOutput::ViewerOutput() :
 
 ViewerOutput::~ViewerOutput()
 {
+  // Should prevent traversing graph unnecessarily
+  BeginOperation();
   DisconnectAll();
+  EndOperation();
 }
 
 Node *ViewerOutput::copy() const

app/project/item/sequence/sequence.cpp

@@ -55,7 +55,10 @@ Sequence::Sequence()
 
 Sequence::~Sequence()
 {
+  // Should prevent traversing graph unnecessarily
+  BeginOperation();
   DisconnectAll();
+  EndOperation();
 }
 
 void Sequence::add_default_nodes(MultiUndoCommand* command)