NPE changing property in function to one with different type

Property dialog should prevent properties with different types from being selected.

java.lang.NullPointerException
at org.jboss.tools.fuse.transformation.editor.internal.MappingDetailViewer.createFunctionSourcePane(MappingDetailViewer.java:258)
at org.jboss.tools.fuse.transformation.editor.internal.MappingDetailViewer.update(MappingDetailViewer.java:590)
at org.jboss.tools.fuse.transformation.editor.TransformationEditor.selected(TransformationEditor.java:494)
at org.jboss.tools.fuse.transformation.editor.internal.MappingsViewer.selected(MappingsViewer.java:228)
at org.jboss.tools.fuse.transformation.editor.internal.MappingSummary.selected(MappingSummary.java:220)
at org.jboss.tools.fuse.transformation.editor.internal.MappingSummary$3.focusGained(MappingSummary.java:170)