Set source/target container figure to null after Zest connection removal

The local variables should be cleared when the figure is removed, even
if either of the containers has already been disposed.

Amends 2288625

org.eclipse.zest.core/src/org/eclipse/zest/core/widgets/GraphConnection.java

@@ -163,12 +163,18 @@ void removeFigure() {
 			}
 		}
 		connectionFigure = null;
-		if (sourceContainerConnectionFigure != null && sourceContainerConnectionFigure.getParent() != null) {
-			sourceContainerConnectionFigure.getParent().remove(sourceContainerConnectionFigure);
+		if (sourceContainerConnectionFigure != null) {
+			IFigure sourceContainerConnectionParent = sourceContainerConnectionFigure.getParent();
+			if (sourceContainerConnectionParent != null) {
+				sourceContainerConnectionParent.remove(sourceContainerConnectionFigure);
+			}
 			sourceContainerConnectionFigure = null;
 		}
 		if (targetContainerConnectionFigure != null && targetContainerConnectionFigure.getParent() != null) {
-			targetContainerConnectionFigure.getParent().remove(targetContainerConnectionFigure);
+			IFigure targetContainerConnectionParent = targetContainerConnectionFigure.getParent();
+			if (targetContainerConnectionParent != null) {
+				targetContainerConnectionParent.remove(targetContainerConnectionFigure);
+			}
 			targetContainerConnectionFigure = null;
 		}