Removing ubuntu-sdk remove the dummy 'ubuntu-sdk` package only

Just find out that ubuntu-sdk* will purge everything. But still I would appreciate an explanation. Thanks :)

Try

sudo apt-get autoremove --purge

Installing ubuntu-sdk will install certain qt-creator packages. and when you remove the ubuntu-stk package, the will not automatically be removed again. They should still be functioning, so if one of the programs crashes when accessing the options dialogue, you definitely have a problem on your system, and that error is independent from (un)installing ubuntu-sdk.

But my Qt crashes when I remove the sdk package. And it runs normal when I installed it back.

This is weird.

To provide some more information about your system please provide the output of the commands

uname -a
lsb_release -crid
sudo dpkg --audit
apt-cache policy qtcreator
apt-get --simulate autoremove

Linux lenovo-g50 4.2.0-18-generic #22-Ubuntu SMP Fri Nov 6 18:25:50 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
Distributor ID: Ubuntu
Description: Ubuntu 15.10
Release: 15.10
Codename: wily

$ sudo dpkg --audit

$ apt-cache policy qtcreator
qtcreator:
  Installed: 3.5.0+dfsg-1ubuntu3
  Candidate: 3.5.0+dfsg-1ubuntu3
  Version table:
 *** 3.5.0+dfsg-1ubuntu3 0
        500 http://archive.ubuntu.com/ubuntu/ wily/universe amd64 Packages
        100 /var/lib/dpkg/status

apt-get --simulate autoremove
.....
The following packages will be REMOVED:
  giblib1 libimlib2 scrot
0 upgraded, 0 newly installed, 3 to remove and 8 not upgraded.
Remv scrot [0.8-16]
Remv giblib1 [1.2.4-10]
Remv libimlib2 [1.4.7-1]

Apparently not all packages are updated to the latest version (but that should not cause program aborts).

What is the output of

apt-get --simulate dist-upgrade

The following packages will be upgraded:
  google-chrome-stable im-config kpartx liblightdm-gobject-1-0
  libtracker-sparql-1.0-0 libwhoopsie0 lightdm whoopsie
8 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Inst google-chrome-stable [44.0.2403.130-1] (46.0.2490.86-1 Google:1.0/stable [amd64])
Inst lightdm [1.16.4-0ubuntu1] (1.16.5-0ubuntu1 Ubuntu:15.10/wily-updates [amd64])
Inst whoopsie [0.2.49] (0.2.49.1 Ubuntu:15.10/wily-updates [amd64]) []
Inst libwhoopsie0 [0.2.49] (0.2.49.1 Ubuntu:15.10/wily-updates [amd64])
Inst im-config [0.29-1ubuntu7] (0.29-1ubuntu7.1 Ubuntu:15.10/wily-updates [all])
Inst liblightdm-gobject-1-0 [1.16.4-0ubuntu1] (1.16.5-0ubuntu1 Ubuntu:15.10/wily-updates [amd64])
Inst libtracker-sparql-1.0-0 [1.4.1-1ubuntu2] (1.4.1-1ubuntu2.1 Ubuntu:15.10/wily-updates [amd64])
Inst kpartx [0.5.0-7ubuntu7] (0.5.0-7ubuntu7.1 Ubuntu:15.10/wily-updates [amd64])
Conf google-chrome-stable (46.0.2490.86-1 Google:1.0/stable [amd64])
Conf lightdm (1.16.5-0ubuntu1 Ubuntu:15.10/wily-updates [amd64])
Conf libwhoopsie0 (0.2.49.1 Ubuntu:15.10/wily-updates [amd64])
Conf whoopsie (0.2.49.1 Ubuntu:15.10/wily-updates [amd64])
Conf im-config (0.29-1ubuntu7.1 Ubuntu:15.10/wily-updates [all])
Conf liblightdm-gobject-1-0 (1.16.5-0ubuntu1 Ubuntu:15.10/wily-updates [amd64])
Conf libtracker-sparql-1.0-0 (1.4.1-1ubuntu2.1 Ubuntu:15.10/wily-updates [amd64])
Conf kpartx (0.5.0-7ubuntu7.1 Ubuntu:15.10/wily-updates [amd64])

There seem to be eight non-security updates waiting to be installed.
I recommend that you install them with

sudo apt-get dist-upgrade

I do not see anything that could be related to program crashes with qt creator.

If the crash with qt designer is repeatable, then you should create a bug report about this problem.