Bug 357538

Steps to reproduce:
1. Start kdenlive
2. Import audio file
3. Change profile to 1080p 30fps
4. Confirm
(If can't be reproduced, consider temporarily removing ~/.config/kdenliverc)

Expected result:
1. No crash

Actual result:
1. Crash

output/bt:
> Starting program: /usr/bin/kdenlive 
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/usr/lib/libthread_db.so.1".
> [New Thread 0x7fffe461e700 (LWP 19099)]
> [New Thread 0x7fffc37a9700 (LWP 19100)]
> [New Thread 0x7fffbe416700 (LWP 19101)]
> [New Thread 0x7fffbdc15700 (LWP 19102)]
> [New Thread 0x7fffbcc14700 (LWP 19103)]
> Removing cache at "/home/qubodup/.cache/kdenlive-thumbs.kcache"
> QCursor: Cannot create bitmap cursor; invalid bitmap(s)
> [New Thread 0x7fffaeae2700 (LWP 19104)]
> Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures.
> [New Thread 0x7fffae2e1700 (LWP 19105)]
> QXcbConnection: XCB error: 8 (BadMatch), sequence: 8023, resource id: 37748957, major code: 131 (Unknown), minor code: 3
> QXcbConnection: XCB error: 8 (BadMatch), sequence: 8048, resource id: 37748957, major code: 131 (Unknown), minor code: 3
> [New Thread 0x7fffac9e0700 (LWP 19106)]
>  / / / / /WARNING, VIDEO CONTEXT IS NULL!!!!!!!!!!!!!!
> [New Thread 0x7fff9ffff700 (LWP 19107)]
> "Creating audio thumbnails (1/1)"
> [New Thread 0x7fff9df20700 (LWP 19108)]
> [Thread 0x7fff9df20700 (LWP 19108) exited]
> [New Thread 0x7fff9df20700 (LWP 19109)]
> [Thread 0x7fff9df20700 (LWP 19109) exited]
> [New Thread 0x7fff9df20700 (LWP 19110)]
> [New Thread 0x7fff93ffe700 (LWP 19111)]
> [New Thread 0x7fff9319e700 (LWP 19112)]
> [New Thread 0x7fff9299d700 (LWP 19113)]
> [New Thread 0x7fff91f8f700 (LWP 19114)]
> [New Thread 0x7fff9178e700 (LWP 19115)]
> [New Thread 0x7fff90f8d700 (LWP 19116)]
> [New Thread 0x7fff83ffd700 (LWP 19117)]
> [Thread 0x7fff9df20700 (LWP 19110) exited]
> QCoreApplication::postEvent: Unexpected null receiver
>  / / / / /WARNING, VIDEO CONTEXT IS NULL!!!!!!!!!!!!!!
> 
> Program received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7fffac9e0700 (LWP 19106)]
> 0x00007ffff6b7d400 in Mlt::Properties::get(char const*) ()
>    from /usr/lib/libmlt++.so.3
> (gdb) bt
> #0  0x00007ffff6b7d400 in Mlt::Properties::get(char const*) ()
>    from /usr/lib/libmlt++.so.3
> #1  0x00000000005773ca in ?? ()
> #2  0x0000000000520c49 in ?? ()
> #3  0x0000000000811ac4 in ?? ()
> #4  0x00007ffff1cfa31a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/libQt5Core.so.5
> #5  0x000000000081c3e1 in ?? ()
> #6  0x00007ffff1cfa31a in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/libQt5Core.so.5
> #7  0x00000000007f8a65 in ?? ()
> #8  0x0000000000750b0b in ?? ()
> #9  0x00000000007d9fe6 in ?? ()
> #10 0x00000000004a808d in ?? ()
> #11 0x00007ffff1ae7683 in ?? () from /usr/lib/libQt5Core.so.5
> #12 0x00007ffff1aeab8e in ?? () from /usr/lib/libQt5Core.so.5
> #13 0x00007fffefaba4a4 in start_thread ()
>    from /usr/lib/libpthread.so.0
> #14 0x00007ffff110a13d in clone () from /usr/lib/libc.so.6

Video (1m44s duration, mostly waiting for gdb session to start):
https://youtu.be/fkgO3R2yHvg