Context Files after xcode crash empty/Persistent data in core Data

Xcode crashed unexpectedly last week. After restarting, Xcode had a problem so I restarted my MacBook. I reopened Xcode and started a simulation and no Data where shown, so I debugged through my code and the Context file seems to be empty. Now I have several questions:
1) Has anybody an idea why this happened ?
2) How can I prevent this problem?
3) Are context files for persistent data savings anyway?

Context Files after xcode crash empty/Persistent data in core Data
 
 
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