Crash during undo/redo of adding a new QQ- or KDE-plot

Requirements

• I searched through the issues if the issue already exists

Steps to reproduce

1. create a QQ or KDE plot
2. undo this step
3. redo

Expected

The application doesn't crash.

Observed

The application is crashing because of the assert in

when trying to insert the first child XYCurve of the QQ plot again (why?).

Labplot Version

Debug build 2.10.0-809-g67b6c10a2
Sep 5 2023, 12:04:59
System: openSUSE Leap 15.4
Locale: English,United States (Decimal point ',', Group separator '.'
Number settings: Decimal point ',', Group separator '.', Exponential 'e', Zero digit '0', Percent '%', Positive/Negative sign '+'/'-' (Updated on restart)
Architecture: x86_64-little_endian-lp64
Kernel: linux 5.14.21-150400.24.81-default
C++ Compiler: GNU 7.5.0
C++ Compiler Flags: -fno-operator-names -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -Werror=init-self -Wvla -Wdate-time -Wsuggest-override -Wlogical-op -Wall -Wextra -Wundef -Wpointer-arith -Wunreachable-code -Wunused -Wdeprecated-declarations -fno-omit-frame-pointer -fstack-protector -fexceptions -std=c++17 -O2 -Wcast-align -Wswitch-enum -fvisibility=default -pedantic -Wzero-as-null-pointer-constant