Add checking for Add command

[weixin-tan]

No description provided.

[codecov-io]

Codecov Report

Merging #144 into master will increase coverage by 0.17%.
The diff coverage is 60.86%.

@@             Coverage Diff              @@
##             master     #144      +/-   ##
============================================
+ Coverage     58.61%   58.78%   +0.17%     
- Complexity      590      591       +1     
============================================
  Files           111      110       -1     
  Lines          2160     2169       +9     
  Branches        244      246       +2     
============================================
+ Hits           1266     1275       +9     
+ Misses          813      811       -2     
- Partials         81       83       +2     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
.../java/seedu/address/logic/commands/AddCommand.java	77.77% <50.00%> (-5.56%)	9.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	66.66% <50.00%> (-0.99%)	33.00 <0.00> (ø)
...n/java/seedu/address/model/order/OrderManager.java	58.69% <60.00%> (-1.77%)	12.00 <1.00> (ø)
src/main/java/seedu/address/model/order/Order.java	88.50% <70.00%> (-2.52%)	32.00 <1.00> (+1.00)	⬇️

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[MorningLit]

lgtm