App: [skip ci] fix possible segmentation fault when performing a transaction after an undo

src/App/Document.cpp

@@ -178,6 +178,7 @@ struct DocumentP
     bool rollback;
     bool undoing; ///< document in the middle of undo or redo
     bool committing;
+    bool opentransaction;
     std::bitset<32> StatusBits;
     int iUndoMode;
     unsigned int UndoMemSize;
@@ -205,6 +206,7 @@ struct DocumentP
         rollback = false;
         undoing = false;
         committing = false;
+        opentransaction = false;
         StatusBits.set((size_t)Document::Closable, true);
         StatusBits.set((size_t)Document::KeepTrailingDigits, true);
         StatusBits.set((size_t)Document::Restoring, false);
@@ -1101,6 +1103,14 @@ int Document::_openTransaction(const char* name, int id)
     }
 
     if (d->iUndoMode) {
+        // Avoid recursive calls that is possible while
+        // clearing the redo transactions and will cause
+        // a double deletion of some transaction and thus
+        // a segmentation fault
+        if (d->opentransaction)
+            return 0;
+        Base::FlagToggler<> flag(d->opentransaction);
+
         if(id && mUndoMap.find(id)!=mUndoMap.end())
             throw Base::RuntimeError("invalid transaction id");
         if (d->activeUndoTransaction)