Reproduction steps:
1. Create a project or open any project
2. Select an object in Hierarchy dialog
3. Try to deselect the object by clicking on it again in Hierarchy dialog while holding either Ctrl or Shift
4. Observe, impossible to deselect the object

Expected result: Object should be deselected
Actual result: Impossible to deselect the object

Reproduced with: 5.5.0a6, 5.5.0f3, 5.5.0p4, 5.6.0b3
Not reproduced with: 5.4.4p1, 5.3.7p3, 5.5.0a5
Regression introduced in 5.5.0a6