Fix missing node offset initialization

src/mappingMatrix.cpp

@@ -95,6 +95,7 @@ class MappingMatrix : public QWidget
 			// we must generate this unique identifier ourselves and also keep the offset to the first input node
 			// in order to be able to translate connections back and forth.
 			auto id = 0;
+			_offset = static_cast<decltype(_offset)>(outputs.size());
 
 			// Create output nodes
 			_outputNodes.reserve(outputs.size());