STORM-494 FIXED Message Well window is undocked while opening Side bar

- To calculate properly whether the dockable floater visible or not, it's needed to get root view as dockable floater's parent

indra/llui/lldockcontrol.cpp

@@ -160,8 +160,10 @@ bool LLDockControl::isDockVisible()
 			case TOP:
 			{
 				// check is dock inside parent rect
+				// assume that parent for all dockable flaoters
+				// is the root view
 				LLRect dockParentRect =
-						mDockWidget->getParent()->calcScreenRect();
+						mDockWidget->getRootView()->calcScreenRect();
 				if (dockRect.mRight <= dockParentRect.mLeft
 						|| dockRect.mLeft >= dockParentRect.mRight)
 				{