K2 Scripting, IR: non-class based scripts support in IR

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/ScriptRemoveReceiverLowering.kt

@@ -45,7 +45,7 @@ class ScriptRemoveReceiverLowering(val context: CommonBackendContext) : FileLowe
             }
 
             override fun visitGetValue(expression: IrGetValue): IrExpression {
-                return if (expression.symbol === script.thisReceiver.symbol) expression.nullConst()
+                return if (expression.symbol === script.thisReceiver?.symbol) expression.nullConst()
                 else expression
             }