Steps to reproduce:
Create a new project->Add new Shapefile Layer(Polygon)->Click on the newly added Layer->Toggle Editing->Using the 'Add Feature' Tool draw a polygon and save the layer
Open the python console->Run the following lines of code in the python console
from PyQt4.QtGUI import QColor
layer = iface.activeLayer()
symbol = QgsLineSymbolV2.createSimple({})
symbol.deleteSymbolLayer(0)
symbol_layer = QgsSimpleLineSymbolLayerV2(QColor('red'), 2.0)
symbol.appendSymbolLayer(symbol_layer.clone())
layer.rendererV2().setSymbol(symbol.clone())
layer.triggerRepaint()
iface.legendInterface().refreshLayerSymbology(layer)
After running the above code the polygon stops displaying. Is this a bug in QGIS 2.18 or am I missing something?