After disconnecting an external monitor(secondary screen), even the primary display goes blank. This gives the user no clues as to what is happening. The worst part being that the problem persists even after rebooting and there's apparently no way to get rid of it. 

Reproducible: Always

Steps to Reproduce:
1. Connect an external screen.
2. Disable the primary screen, continue working on the primary screen.
3. Disconnect the external screen.

Actual Results:  
The primary screen goes blank ! 

Expected Results:  
The primary screen should be active once the external screen is disconnected.

Backtrace obtained on kubuntu 14.10(ppa:kubuntu/ci) 

Steps to Reproduce:
1. Connect to an external screen.
2. Disable the primary screen, keep working on the secondary screen.
3. Disconnect the external screen.

Thread 1 (Thread 0x7ffff7e48880 (LWP 19845)):
#0  QScreen::handle (this=this@entry=0x0) at kernel/qscreen.cpp:85
#1  0x00007fffe64befc2 in QXcbNativeInterface::nativeResourceForScreen (this=<optimized out>, resource=..., screen=0x0) at qxcbnativeinterface.cpp:257
#2  0x00007ffff7f4c9bb in QX11Info::display() () from /usr/lib/x86_64-linux-gnu/libQt5X11Extras.so.5
#3  0x00000000004456dc in PanelShadows::Private::clearShadow (window=0xa9fe40, this=<optimized out>) at ../../shell/panelshadows.cpp:493
#4  0x0000000000448445 in clearShadow (window=<optimized out>, this=<optimized out>) at ../../shell/panelshadows.cpp:147
#5  PanelShadows::removeWindow (this=0x697aa0 <(anonymous namespace)::Q_QGS_privateDialogShadowsSelf::innerFunction()::holder>, window=window@entry=0xa9fe40)
    at ../../shell/panelshadows.cpp:142
#6  0x000000000043facf in PanelView::~PanelView (this=0xa9fe40, __in_chrg=<optimized out>) at ../../shell/panelview.cpp:124
#7  0x000000000043fb49 in PanelView::~PanelView (this=0xa9fe40, __in_chrg=<optimized out>) at ../../shell/panelview.cpp:125
#8  0x000000000044f4c4 in ShellCorona::removeView (this=0x700770, idx=0) at ../../shell/shellcorona.cpp:695
#9  0x000000000044f549 in ShellCorona::screenRemoved (this=0x700770, screen=<optimized out>) at ../../shell/shellcorona.cpp:728
#10 0x00007ffff352efea in call (a=0x7fffffffdf80, r=0x700770, this=0x9be270) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:124
#11 QMetaObject::activate (sender=0x7fffffffea00, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=2, argv=argv@entry=0x7fffffffdf80)
    at kernel/qobject.cpp:3702
#12 0x00007ffff352fce7 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7ffff3c8e900 <QGuiApplication::staticMetaObject>, 
    local_signal_index=local_signal_index@entry=2, argv=argv@entry=0x7fffffffdf80) at kernel/qobject.cpp:3582
#13 0x00007ffff383f5e2 in QGuiApplication::screenRemoved (this=<optimized out>, _t1=_t1@entry=0x6c69b0) at .moc/moc_qguiapplication.cpp:328
#14 0x00007ffff386efe9 in QScreen::~QScreen (this=0x6c69b0, __in_chrg=<optimized out>) at kernel/qscreen.cpp:76
#15 0x00007ffff386f009 in QScreen::~QScreen (this=0x6c69b0, __in_chrg=<optimized out>) at kernel/qscreen.cpp:77
#16 0x00007ffff3836a42 in QPlatformScreen::~QPlatformScreen (this=0x6c69d0, __in_chrg=<optimized out>) at kernel/qplatformscreen.cpp:57
#17 0x00007fffe64b33a9 in QXcbScreen::~QXcbScreen (this=0x6c69d0, __in_chrg=<optimized out>) at qxcbscreen.cpp:211
#18 0x00007fffe64a63f7 in QXcbConnection::updateScreens (this=this@entry=0x6b8b90) at qxcbconnection.cpp:286
#19 0x00007fffe64a70bd in QXcbConnection::handleXcbEvent (this=this@entry=0x6b8b90, event=event@entry=0x7fffdc00a6f0) at qxcbconnection.cpp:955
#20 0x00007fffe64a835b in QXcbConnection::processXcbEvents (this=0x6b8b90) at qxcbconnection.cpp:1297
#21 0x00007ffff35307ba in QObject::event (this=0x6b8b90, e=<optimized out>) at kernel/qobject.cpp:1245
#22 0x00007ffff3dfed3c in QApplicationPrivate::notify_helper (this=0x6ae220, receiver=0x6b8b90, e=0x7fffdc00a8e0) at kernel/qapplication.cpp:3722
#23 0x00007ffff3e04330 in QApplication::notify (this=0x7fffffffea00, receiver=0x6b8b90, e=0x7fffdc00a8e0) at kernel/qapplication.cpp:3505
#24 0x00007ffff350003b in QCoreApplication::notifyInternal (this=0x7fffffffea00, receiver=0x6b8b90, event=event@entry=0x7fffdc00a8e0) at kernel/qcoreapplication.cpp:932
#25 0x00007ffff350207b in sendEvent (event=0x7fffdc00a8e0, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:228
#26 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x6a5100) at kernel/qcoreapplication.cpp:1536
#27 0x00007ffff3502678 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1394
#28 0x00007ffff3556e83 in postEventSourceDispatch (s=0x6dd350) at kernel/qeventdispatcher_glib.cpp:271
#29 0x00007fffef577c5d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#30 0x00007fffef577f48 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#31 0x00007fffef577ffc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007ffff3557277 in QEventDispatcherGlib::processEvents (this=0x6dcf20, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#33 0x00007ffff34fd8d2 in QEventLoop::exec (this=this@entry=0x7fffffffe8a0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#34 0x00007ffff350537c in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1185
#35 0x00007ffff3840bcc in QGuiApplication::exec () at kernel/qguiapplication.cpp:1535
#36 0x00007ffff3dfadf5 in QApplication::exec () at kernel/qapplication.cpp:2956
#37 0x000000000042fcaf in main (argc=1, argv=<optimized out>) at ../../shell/main.cpp:150