1. Every year got people predict stock market crash. No matter market crash or not, you should always have a warchest ready.
2. Even if market crash there are always winners and losers. Instead of trying to time the market, you should just invest in fundamentally strong companies.
3. Even if you did get the timing correct and have mostly cash in hand, there are only a very few that can buy right at market bottom. Chances are you'll buy way too early and stock sank another 40% after the initial 20% decline or simply never enter because of fear.
Final word, if you think stocks are in bubble, then invest those that are undervalued or unliked at present.