Add Data Workflow "back" button causes Exception

[fgdrf]

Workflow to reproduce:

• Menu -> Layer -> Add ..
• Choose e.g. Files
• Abort the FileDialog
• Now "Back" Button is enabled, if hit the following exception is logged:

java.lang.NullPointerException
	at org.locationtech.udig.catalog.ui.workflow.WorkflowWizardDialog.backPressed(WorkflowWizardDialog.java:157)
	at org.eclipse.jface.wizard.WizardDialog.buttonPressed(WizardDialog.java:406)
	at org.eclipse.jface.dialogs.Dialog.lambda$0(Dialog.java:622)
	at org.eclipse.swt.events.SelectionListener$1.widgetSelected(SelectionListener.java:84)
	at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:252)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4131)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1055)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3944)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3547)
	at org.eclipse.jface.window.Window.runEventLoop(Window.java:823)
	at org.eclipse.jface.window.Window.open(Window.java:799)
	at org.locationtech.udig.catalog.ui.workflow.WorkflowWizardDialog.open(WorkflowWizardDialog.java:220)
	at org.locationtech.udig.project.ui.internal.actions.AddLayersAction.runWithEvent(AddLayersAction.java:50)
	at org.eclipse.ui.internal.PluginAction.runWithEvent(PluginAction.java:240)
	at org.eclipse.ui.internal.WWinPluginAction.runWithEvent(WWinPluginAction.java:230)
	at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:568)
	at org.eclipse.jface.action.ActionContributionItem.lambda$4(ActionContributionItem.java:400)
	at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
	at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4131)
	at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1055)
	at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3944)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3547)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$5.run(PartRenderingEngine.java:1173)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:338)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1062)
	at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:155)
	at org.eclipse.ui.internal.Workbench.lambda$3(Workbench.java:644)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:338)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:566)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:150)
	at org.locationtech.udig.internal.ui.UDIGApplication.start(UDIGApplication.java:135)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:203)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:137)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:107)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:400)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:255)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:661)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:597)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1476)

Expectation: Back should not be enabled or no Exception is thrown

[fgdrf]

this issue is related to #577