script interpreter gracefully handling null result (#3832)

fixes #3831
Signed-off-by: Simon Kaufmann <simon.kfm@googlemail.com>

bundles/model/org.eclipse.smarthome.model.script/src/org/eclipse/smarthome/model/script/interpreter/ScriptInterpreter.xtend

@@ -23,6 +23,7 @@ import org.eclipse.xtext.xbase.interpreter.IEvaluationContext
 import org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter
 import org.eclipse.xtext.xbase.jvmmodel.IJvmModelAssociations
 import org.eclipse.xtext.common.types.JvmIdentifiableElement
+import org.eclipse.xtext.xbase.XExpression
 
 /**
  * The script interpreter handles ESH specific script components, which are not known
@@ -111,5 +112,12 @@ public class ScriptInterpreter extends XbaseInterpreter {
 			super._assignValueTo(jvmField, assignment, value, context, indicator)
 		}
 	}
+
+    override protected doEvaluate(XExpression expression, IEvaluationContext context, CancelIndicator indicator) {
+        if (expression == null) {
+            return null
+        }
+        return super.doEvaluate(expression, context, indicator)
+    }
 
 }