Fix NPE in ExpandFunctionDefinitionConverter for getDerivedUnits (issue #268)

core/src/org/sbml/jsbml/util/converters/ExpandFunctionDefinitionConverter.java

@@ -237,11 +237,16 @@ public boolean accepts(Object o) {
           ASTNode current = (ASTNode) o;
 
           if (current.equals(bvar)) {
-            ASTNode parent = (ASTNode) current.getParent();
-            int index = parent.getIndex(current);
+            TreeNode parentNode = current.getParent();
 
-            if (index != -1) {
-              parent.replaceChild(index, expandedBVar);
+            // Only attempt replacement if the parent is a non-null ASTNode
+            if (parentNode instanceof ASTNode) {
+              ASTNode parent = (ASTNode) parentNode;
+              int index = parent.getIndex(current);
+
+              if (index != -1) {
+                parent.replaceChild(index, expandedBVar);
+              }
             }
           }
         }

core/test/org/sbml/jsbml/AssignmentRuleDerivedUnitsTest.java

@@ -0,0 +1,50 @@
+package org.sbml.jsbml;
+
+import static org.junit.Assert.*;
+
+import java.io.File;
+import java.net.URL;
+
+import org.junit.Test;
+
+public class AssignmentRuleDerivedUnitsTest {
+
+  /**
+   * Regression test for issue #268.
+   * Calling getDerivedUnits() on an AssignmentRule in BIOMD0000000327.xml
+   * should not throw a NullPointerException.
+   */
+  @Test
+  public void testGetDerivedUnitsDoesNotThrowNPE() throws Exception {
+    // Load the example model from test resources
+    ClassLoader cl = getClass().getClassLoader();
+    URL url = cl.getResource("org/sbml/jsbml/testdata/BIOMD0000000327.xml");
+    assertNotNull("Test SBML file not found on classpath", url);
+
+    SBMLDocument doc = new SBMLReader().readSBML(new File(url.toURI()));
+    Model model = doc.getModel();
+    assertNotNull(model);
+
+    // Find the assignment rule for variable "japl"
+    AssignmentRule targetRule = null;
+    for (int i = 0; i < model.getRuleCount(); i++) {
+    Rule r = model.getRule(i);
+    if (r instanceof AssignmentRule) {
+        AssignmentRule ar = (AssignmentRule) r;
+        if ("japl".equals(ar.getVariable())) {
+        targetRule = ar;
+        break;
+        }
+    }
+    }
+
+    assertNotNull("AssignmentRule for variable 'japl' not found", targetRule);
+
+    // The call to getDerivedUnits() must not throw a NullPointerException
+    try {
+      targetRule.getDerivedUnits();
+    } catch (NullPointerException npe) {
+      fail("getDerivedUnits() must not throw NullPointerException: " + npe);
+    }
+  }
+}