[kernel] Add assertions for testing NPE.

Signed-off-by: Stéphane Galland <galland@arakhne.org>

io.janusproject.kernel/src/main/java/io/janusproject/kernel/services/jdk/contextspace/StandardContextSpaceService.java

@@ -202,16 +202,20 @@ public synchronized boolean containsContext(UUID contextID) {
 	 */
 	@Override
 	public synchronized AgentContext createContext(UUID contextID, UUID defaultSpaceUUID) {
+		assert (contextID != null) : "The contextID cannot be null"; //$NON-NLS-1$
+		assert (defaultSpaceUUID != null) : "The defaultSpaceUUID cannot be null"; //$NON-NLS-1$
 		AgentContext context = this.contexts.get(contextID);
 		if (context == null) {
 			Context ctx = this.contextFactory.newInstance(
 					contextID,
 					defaultSpaceUUID,
 					this.spaceRepositoryFactory,
 					new SpaceEventProxy());
+			assert (ctx != null) : "The internal Context cannot be null"; //$NON-NLS-1$
 			this.contexts.put(contextID, ctx);
 			fireContextCreated(ctx);
 			Space defaultSpace = ctx.postConstruction();
+			assert (defaultSpace != null) : "The default space in the context " + contextID + " cannot be null"; //$NON-NLS-1$ //$NON-NLS-2$
 			this.defaultSpaces.putIfAbsent(ctx.getID(), defaultSpace.getID());
 			return ctx;
 		}