SimpleNode#evaluateGetValueBody(OgnlContext, Object) might return null.
I have a patch that resolves the issue for version 2.6.9. Same issue might be in the above vesions.

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Index: C:/User/Work/workspace/ognl/src/java/ognl/SimpleNode.java
===================================================================
--- C:/User/Work/workspace/ognl/src/java/ognl/SimpleNode.java (revision 139)
+++ C:/User/Work/workspace/ognl/src/java/ognl/SimpleNode.java (working copy)
@@ -44,7 +44,7 @@
     protected OgnlParser parser;
 
     private boolean constantValueCalculated;
- private boolean hasConstantValue;
+ private volatile boolean hasConstantValue;
     private Object constantValue;
 
     public SimpleNode(int i) {
@@ -162,10 +162,11 @@
         context.setCurrentNode(this);
         if (!constantValueCalculated) {
             constantValueCalculated = true;
- hasConstantValue = isConstant(context);
- if (hasConstantValue) {
+ boolean constant = isConstant(context);
+ if (constant) {
                 constantValue = getValueBody(context, source);
             }
+ hasConstantValue = constant;
         }
         return hasConstantValue ? constantValue : getValueBody(context, source);
     }
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