You can incorporate later, but the decisions you make as a sole proprietor or sole trader can come back to haunt you even after incorporation. Your liability after incorporating will depend on the nature of the app and the way you bring it into your corporation. If damage results from a problem that a lawyer can demonstrate pre-existed and should have reasonably been taken care of before incorporation, you will be in his cross-hairs.

The extent of your liability will depend on where you live (the UK is harsher on developer responsibilities, IMO). IANAL, but I have worked as both sole trader and company director. Incorporation is dirt cheap compared to the cost of liability.