You are using tabs, please use spaces. Check how it looks: 
https://github.com/topilski/robomongo/blob/master/src/robomongo/gui/GuiRegistry.h#L41

This rule is documented here:
https://github.com/paralect/robomongo/wiki/Robomongo-Coding-Style#wiki-no-tabs

Also, put space before `const`. You can see existing style here:
https://github.com/topilski/robomongo/blob/master/src/robomongo/gui/GuiRegistry.h#L28

```cpp
MainWindow *mainWindow() const { return _mainWindow;}
```