An error occurred while executing a save: Expression evaluation error at function fn!fromJsonWcf_appian_internal

I am getting this error: 

An error occurred while executing a save: Expression evaluation error at function fn!fromJsonWcf_appian_internal [line 46]: java.lang.IllegalArgumentException: com.fasterxml.jackson.core.JsonParseException: Unexpected character ('!' (code 33)): expected a valid value (number, String, array, object, 'true', 'false' or 'null') at [Source: (String)"[!_{"h":""}]"; line: 1, column: 3]

when using custom SAIL plugin.

 My custom plugin doesn't give any output except HTML design.

This happens only on the first time whenever my interface/site loads if I change to expression mode or switch between tabs (In site), the error disappears and plugin works normally.

I am not sure what is this error and why this is happening.

Any help would be appreciated.