Robin Williams was constantly trying to shake off an "invisible monster" as he struggled with Lewy body dementia, according to the comedy icon's widow.Susan Schneider Williams has opened up about the debilitating illness which led the Mrs. Doubtfire star to take his own life in new documentary Robin's Wish, revealing they always knew there was more to his health crisis than the Parkinson's disease he had been diagnosed with just three months before his death in August 2014.