On pc Debian x86-64 with master sources updated today, I don't reproduce this with gtk3 or gen rendering.
Windows only?

I can't reproduce it with:
Version: 6.5.0.0.alpha0+ (x64)
Build ID: 350d25da375f221edfa37309324ce3c68cf297ef
CPU threads: 4; OS: Windows 10.0 Build 18362; UI render: GL; VCL: win; 
Locale: pl-PL (pl_PL); UI-Language: en-US
Calc: CL

And as you said, no crash with LO 6.4.0.2.

(In reply to Dagmara from comment #2)
> I can't reproduce it with:
> Version: 6.5.0.0.alpha0+ (x64)
> Build ID: 350d25da375f221edfa37309324ce3c68cf297ef

Have you tried with a *Debug* Build ?
I can still reproduce after a clean and full build with:

Version: 6.5.0.0.alpha0+ (x64)
Build ID: 33ac8ea863ddba903834fe9d0e7165a087bb9c5f
CPU threads: 12; OS: Windows 10.0 Build 18363; UI render: GL; VCL: win; 
Locale: de-DE (de_DE); UI-Language: en-US
Calc: CL

> warn:legacy.osl:5228:8952:vcl/source/window/window.cxx:269: 
> Window ( class VclVBox()) with live children destroyed:  class 
> FormattedField(0) class FormattedField()
> Window ( class VclVBox()) with live children destroyed:  class 
> FormattedField(0) class FormattedField()

#if OSL_DEBUG_LEVEL > 0
    if ( true ) // always perform these tests in debug builds
    {
        OStringBuffer aErrorStr;
        bool        bError = false;
        vcl::Window*     pTempWin;

        if ( mpWindowImpl->mpFirstChild )
        {
            OStringBuffer aTempStr("Window (");
            aTempStr.append(lcl_createWindowInfo(this));
            aTempStr.append(") with live children destroyed: ");
            pTempWin = mpWindowImpl->mpFirstChild;
            while ( pTempWin )
            {
                aTempStr.append(lcl_createWindowInfo(pTempWin));
                pTempWin = pTempWin->mpWindowImpl->mpNext;
            }
269 ->      OSL_FAIL( aTempStr.getStr() );
            Application::Abort(OStringToOUString(aTempStr.makeStringAndClear(),
 RTL_TEXTENCODING_UTF8));   // abort in debug builds, this must be fixed!
        }

no crash in

Версия: 6.5.0.0.alpha0+
ID сборки: b6295e4a1b7735c148174f44f6d28221f4f52302
Потоков ЦП: 4; ОС:Mac OS X 10.15.2; Отрисовка ИП: GL; VCL: osx; 
Локаль: ru-RU (ru_RU.UTF-8); Язык интерфейса: ru-RU
Calc: threaded

Regina: noticing http://document-foundation-mail-archive.969070.n3.nabble.com/Perhaps-crash-in-debug-build-tt4272227.html, thought you might be interested in this one.

I have made a clean build now and test it in Writer. Insert push bottom, context menu, click on item Control properties. The dialog frame opens with title bar and empty content area and crashes then.

Version: 6.5.0.0.alpha0+ (x64)
Build ID: 5434688e5c38d438444dd8d088dff9d3de58214a
CPU threads: 8; OS: Windows 10.0 Build 18362; UI render: GL; VCL: win; 
Locale: en-US (en_US); UI-Language: en-US
Calc: CL

Created attachment 157204 [details]
Visual Studio "Aufrufliste"

Visual Studio error message: Ausgelöste Ausnahme: Lesezugriffsverletzung
"xBuilder._Mypair.**_Myval2**" war "nullptr".

The Visual Studio "Aufrufliste" is attached.

Thank you Regina for your feedback.

I can't reproduce it in

Version: 6.5.0.0.alpha0+
Build ID: cc36c115004cb75037b0e4fdadff09035f765cd5
CPU threads: 4; OS: Linux 4.19; UI render: default; VCL: x11; 
Locale: en-US (en_US.UTF-8); UI-Language: en-US
Calc: threaded

@Oliver, is it related to bug 129484 ? Could you please try with a recent master build ?

It's Windows only, and Regina could reproduce it with a fresh debug build, and attached backtrace. I could also reproduce it with a debug build from a week ago.

(In reply to Xisco Faulí from comment #9)
> @Oliver, is it related to bug 129484 ? Could you please try with a recent
> master build ?

Version: 6.5.0.0.alpha0+ (x64)
Build ID: f7e013657df0a1955d901d79d6df806e5007a9f8
CPU threads: 12; OS: Windows 10.0 Build 18363; UI render: default; VCL: win; 
Locale: de-DE (de_DE); UI-Language: en-US
Calc: CL

will crash selecting "Control Properties..." or "Form Properties..." but not with "Position and Size..."

Created attachment 157231 [details]
bt Windows (windbg)

On Win10 with master sources updated today, I could reproduce this.

I attached a bt which corresponds to what Regina indicated + console logs retrieved from WinDbg

Not reproduced in

Version: 6.5.0.0.alpha0+ (x64)
Build ID: c7f3ac890526306804ec2bdf1256ba15a4247f13
CPU threads: 16; OS: Windows 6.3 Build 9600; UI render: default; VCL: win; 
Locale: en-GB (en_GB); UI-Language: en-US
Calc: threaded

Let change the summary from crash to assert

Mike fixed a bug in this area, just this still reproduce or did he fix this with e.g. b0d2ef6c458a6f216f7ba1683f5c996087644361

I'm not getting a crash/assert in a 7.0.0.0.alpha0+ ( 6388c578c672690fff662cb04b6a0436cd742f37) debug build in Windows. Closing as WFM.