KDevelop Language support C/C++ compilers UI crashes when clicked on manually added compiler.

* How to reproduce? *
1. Start KDevelop, manually add Clang compiler (Settings=>Language support=>C/C++ compilers)
2. Close KDevelop
3. Start KDevelop
4. Open "Settings=>Language support=>C/C++ compilers"
5. Click on previously added compiler.

* What happens? *
- KDevelop crashes.

* Expected behavior? *
KDevelop should allow to edit the compiler record.

* Observations / Notes*

- KDevelop crashes on the line "kdevelop/plugins/custom-definesandincludes/compilerprovider/widget/compilerswidget.cpp:177".
It looks it crashes on calling "indexes.first()" event there is an assert two lines above guarding the "indexes" not to be empty.

- Backtrace shows following locals:

```code
(gdb) info local
indexes = {<QListSpecialMethods<QModelIndex>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, <No data fields>}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x152eb80}, d = 0x152eb80}}                          
compiler = <optimized out>   
```

- It happens at least in KDevelop 5.1.x to 5.2.x including my home build made from recent git sources.

- Also please see attached video for how to reproduce and some GDB output.

- Anyway it is not clear what is this Compilers for. ?

- Also do not understand why it always selects GCC as autodetected when my system uses Clang as default. ?