Allow to set diagram concepts to null

- The corresponding methods setArchimateElement() and setArchimateRelationship() can be set to null so these should allow it too

- Note that eUnset() will set setArchimateElement() and setArchimateRelationship() to null and this can be called by EMF Compare, for example

com.archimatetool.model/src/com/archimatetool/model/impl/DiagramModelArchimateConnection.java

@@ -146,7 +146,7 @@ public IArchimateRelationship getArchimateConcept() {
      */
     @Override
     public void setArchimateConcept(IArchimateConcept concept) {
-        if(!(concept instanceof IArchimateRelationship)) {
+        if(concept != null && !(concept instanceof IArchimateRelationship)) {
             throw new IllegalArgumentException("Should be of type IArchimateRelationship"); //$NON-NLS-1$
         }
         setArchimateRelationship((IArchimateRelationship)concept);

com.archimatetool.model/src/com/archimatetool/model/impl/DiagramModelArchimateObject.java

@@ -357,7 +357,7 @@ public IArchimateElement getArchimateConcept() {
      */
     @Override
     public void setArchimateConcept(IArchimateConcept concept) {
-        if(!(concept instanceof IArchimateElement)) {
+        if(concept != null && !(concept instanceof IArchimateElement)) {
             throw new IllegalArgumentException("Should be of type IArchimateElement"); //$NON-NLS-1$
         }
         setArchimateElement((IArchimateElement)concept);

tests/com.archimatetool.model.tests/src/com/archimatetool/model/impl/DiagramModelArchimateConnectionTests.java

@@ -87,6 +87,12 @@ public void testReconnect() {
     public void testGetArchimateRelationship() {
         assertSame(relationship, connection.getArchimateRelationship());
     }
+
+    @Test
+    public void testSetArchimateRelationshipCanBeNull() {
+        connection.setArchimateRelationship(null);
+        assertNull(connection.getArchimateRelationship());
+    }
 
     @Test
     public void testGetArchimateConcept() {
@@ -100,6 +106,20 @@ public void testSetArchimateConcept() {
         assertSame(r, connection.getArchimateConcept());
         assertSame(r, connection.getArchimateRelationship());
     }
+
+    @Test
+    public void testSetArchimateConceptCanBeNull() {
+        connection.setArchimateConcept(null);
+        assertNull(connection.getArchimateConcept());
+        assertNull(connection.getArchimateRelationship());
+    }
+
+    @Test
+    public void testSetArchimateConceptThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> {
+            connection.setArchimateConcept(IArchimateFactory.eINSTANCE.createBusinessActor());
+        });
+    }
 
     @Test
     public void testAddArchimateRelationshipToModel_AlreadyHasParent() {

tests/com.archimatetool.model.tests/src/com/archimatetool/model/impl/DiagramModelArchimateObjectTests.java

@@ -65,6 +65,12 @@ public void testGetArchimateElement() {
         assertSame(element, object.getArchimateElement());
     }
 
+    @Test
+    public void testSetArchimateElementCanBeNull() {
+        object.setArchimateElement(null);
+        assertNull(object.getArchimateElement());
+    }
+
     @Test
     public void testGetArchimateConcept() {
         assertSame(element, object.getArchimateConcept());
@@ -77,6 +83,20 @@ public void testSetArchimateConcept() {
         assertSame(e, object.getArchimateConcept());
         assertSame(e, object.getArchimateElement());
     }
+
+    @Test
+    public void testSetArchimateConceptCanBeNull() {
+        object.setArchimateConcept(null);
+        assertNull(object.getArchimateConcept());
+        assertNull(object.getArchimateElement());
+    }
+
+    @Test
+    public void testSetArchimateConceptThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> {
+            object.setArchimateConcept(IArchimateFactory.eINSTANCE.createAssociationRelationship());
+        });
+    }
 
     @Test
     public void testGetType() {