Fix bugs

[derekjxtan]

fix app crash when:

1. preferences have null values
2. log file is read only

[codecov[bot]]

Codecov Report

Merging #328 (c1d7370) into master (290d654) will decrease coverage by 0.06%. The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #328      +/-   ##
============================================
- Coverage     80.69%   80.64%   -0.06%     
  Complexity      940      940              
============================================
  Files           137      137              
  Lines          2839     2841       +2     
  Branches        277      278       +1     
============================================
  Hits           2291     2291              
- Misses          513      515       +2     
  Partials         35       35              

Files	Coverage Δ
...in/java/seedu/address/commons/core/LogsCenter.java	78.78% <0.00%> (ø)
src/main/java/seedu/address/MainApp.java	0.00% <0.00%> (ø)

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