Closed Issue: Exception in IsInDesignModeNet() in WP8 app [7671]

Just a minor issue, when using the PCL version of MvvmLight 5.0.2.0 in WP8 app, the ViewModelBase.IsInDesignModeStatic getter throws inner ArgumentNullException. For people with OCD, that don't like Exception logs in the Output windows this could be an issue :)

Problem is in ViewModelBase.cs, line 207 and it can be fixed like this:
```
// now
var dmp = dm.GetTypeInfo().GetDeclaredField("IsInDesignModeProperty").GetValue(null);

// fixed
if (dm == null) return false;
var dmp = dm.GetTypeInfo().GetDeclaredField("IsInDesignModeProperty").GetValue(null);
```

This is probably not the only place where similar exception could happen in this file. It's also better performance-wise to handle these errors manually and not using one big try/catch statement.