adds null check and throws EvaluationException when accessing undefin…

…ed structure values (#350)

adds null check and throws EvaluationException when accessing undefined structure values

src/main/java/com/ezylang/evalex/Expression.java

@@ -186,9 +186,14 @@ private EvaluationValue evaluateArrayIndex(ASTNode startNode) throws EvaluationE
 
   private EvaluationValue evaluateStructureSeparator(ASTNode startNode) throws EvaluationException {
     EvaluationValue structure = evaluateSubtree(startNode.getParameters().get(0));
-    String name = startNode.getParameters().get(1).getToken().getValue();
+    Token nameToken = startNode.getParameters().get(1).getToken();
+    String name = nameToken.getValue();
 
     if (structure.isStructureValue()) {
+      if (!structure.getStructureValue().containsKey(name)) {
+        throw new EvaluationException(
+            nameToken, String.format("Field '%s' not found in structure", name));
+      }
       return structure.getStructureValue().get(name);
     } else {
       throw EvaluationException.ofUnsupportedDataTypeInOperation(startNode.getToken());

src/test/java/com/ezylang/evalex/ExpressionEvaluatorStructureTest.java

@@ -64,4 +64,17 @@ void testThrowsUnsupportedDataTypeForStructure() {
         .isInstanceOf(EvaluationException.class)
         .hasMessage("Unsupported data types in operation");
   }
+
+  @Test
+  void testThrowsFieldNotFound() {
+    Map<String, BigDecimal> testStructure = new HashMap<>();
+    testStructure.put("field1", new BigDecimal(3));
+
+    assertThatThrownBy(
+            () -> createExpression("a.field1 + a.field2").with("a", testStructure).evaluate())
+        .isInstanceOf(EvaluationException.class)
+        .hasMessage("Field 'field2' not found in structure")
+        .extracting("startPosition")
+        .isEqualTo(14);
+  }
 }