When removing a DynamicMapLayer that is outside the min/max zoom levels, the _currentImage is null.  Added a check to prevent a accessing a null value.
When removing a DynamicMapLayer that is outside the min/max zoom levels, the _currentImage is null.  Added a check to prevent a accessing a null value.

jgravois/esri-leaflet,jgravois/esri-leaflet,jgravois/esri-leaflet