Can't drag or add any widgets onto mainwindow.ui in design mode

  • Hi all,
    I'm using Linux Mint 17.2. I haven't used QT Creator for a year or two. I installed the version from the package manager and created a new widgets application and discovered I was unable to drag any widgets on the new main window. I opened up one of my old projects and noticed I could drag around the existing widgets but again I was unable to add any new ones.
    I uninstalled this version of QT Creator and downloaded and installed the latest open source version (3.5.0) from the QT website.
    I still have the same problem. I feel like I'm going bananas because I haven't found anyone else with the same issue yet.
    Any assistance would be greatly appreciated.