fix 2 errors

after deletion of a node:

ButtleOFX/buttleofx/gui/plugin/qml/PluginWindow.qml:241: TypeError:
Cannot read property 'nodeType' of undefined
ButtleOFX/buttleofx/gui/plugin/qml/PluginWindow.qml:261: TypeError:
Cannot read property 'pluginDoc' of undefined
ButtleOFX/buttleofx/gui/plugin/qml/PluginWindow.qml:251: TypeError:
Cannot read property 'pluginGroup' of undefined

and
ButtleOFX/buttleofx/gui/graph/qml/GraphEditor.qml:45: TypeError: Cannot
read property 'coord' of undefined
ButtleOFX/buttleofx/gui/graph/qml/GraphEditor.qml:46: TypeError: Cannot
read property 'coord' of undefined