Bug 574057: Prevent null set of selected data volumes

Prefer an empty set instead.

Change-Id: I6b13f78ffbbbe0cbac2b69ad50abdc90f15fed66
diff --git a/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyTab.java b/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyTab.java
index 4164122..9c5396f 100644
--- a/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyTab.java
+++ b/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyTab.java
@@ -911,7 +911,7 @@
 		}
 		imageCombo.setText(""); //$NON-NLS-1$
 		model.setDataVolumes(null);
-		model.setSelectedDataVolumes(null);
+		model.setSelectedDataVolumes(new HashSet<>());
 		enableButton.setSelection(false);
 		setControlsEnabled(false);
 	}
diff --git a/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyVolumesModel.java b/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyVolumesModel.java
index bbcb6ca..b520dcd 100644
--- a/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyVolumesModel.java
+++ b/launch/org.eclipse.cdt.docker.launcher/src/org/eclipse/cdt/internal/docker/launcher/ContainerPropertyVolumesModel.java
@@ -21,6 +21,7 @@
 import java.util.Set;
 
 import org.eclipse.core.databinding.observable.list.WritableList;
+import org.eclipse.core.runtime.Assert;
 import org.eclipse.linuxtools.docker.core.DockerException;
 import org.eclipse.linuxtools.docker.core.IDockerConnection;
 import org.eclipse.linuxtools.docker.core.IDockerImage;
@@ -129,7 +130,12 @@
 		return selectedDataVolumes;
 	}
 
+	/**
+	 *
+	 * @param selectedDataVolumes non-<code>null</code> set of selected data volumes
+	 */
 	public void setSelectedDataVolumes(final Set<DataVolumeModel> selectedDataVolumes) {
+		Assert.isNotNull(selectedDataVolumes);
 		firePropertyChange(SELECTED_DATA_VOLUMES, this.selectedDataVolumes,
 				this.selectedDataVolumes = selectedDataVolumes);
 	}