Fix NPE

refs 2663

org.fortiss.af3.platform.hierarchic/trunk/src/org/fortiss/af3/platform/hierarchic/compose/elements/MemoryCompositor.java

@@ -50,16 +50,22 @@ public final class MemoryCompositor extends MemoryUnitCompositorBase<Memory> {
 	/** Returns whether the given contained element is of an acceptable class. */
 	@Override
 	protected boolean isCorrectElementClass(Memory container, IArchitectureDomain contained) {
-		// Determine if element can be added, i.e. whether current drag&drop operation
-		// can be finished.
-		return
-		// Elements from the next (more fine-grained) level, that can be dropped into
-		// the model element under the regime of this compositor
-		(getPlatformCompositionRules().isValidContainer(container.getClass(), contained.getClass()) && (canBeDroppedInto(contained) || canBeAddedAtCurrentLevel(contained))) ||
+		if(container != null) {
+			// Determine if element can be added, i.e. whether current drag&drop operation
+			// can be finished.
+			return
+			// Elements from the next (more fine-grained) level, that can be dropped into
+			// the model element under the regime of this compositor
+			(getPlatformCompositionRules().isValidContainer(container.getClass(),
+					contained.getClass()) && (canBeDroppedInto(contained) || canBeAddedAtCurrentLevel(contained))) ||
 
-				// Elements from the same level as the model element
-				// under the regime of this compositor that can be added
-				(getPlatformCompositionRules().hasEqualLevel(container.getClass(),
-						contained.getClass()) && canBeAddedAtCurrentLevel(contained));
+					// Elements from the same level as the model element
+					// under the regime of this compositor that can be added
+					(getPlatformCompositionRules().hasEqualLevel(container.getClass(),
+							contained.getClass()) && canBeAddedAtCurrentLevel(contained));
+		}
+
+		// Determine if element is contributed to current library.
+		return isAvailableInLibrary(contained);
 	}
 }