Editor: Saved random map does segfault on load if no tribe is explicitly set

Steps to reproduce:

1. Start Editor
2. create random map with default values
3. Check player positions. The player menu shows for all players: "Random Player; Tribe Barbarian". Do not change the tribe!
4. Save the map
5. Load the saved map

Result: Widelands exits with a segfault. gdb says:

Thread 1 "widelands" received signal SIGSEGV, Segmentation fault.
0x0000555556105d8a in std::_Rb_tree<unsigned char, unsigned char, std::_Identity<unsigned char>, std::less<unsigned char>, std::allocator<unsigned char> >::_M_begin
    (this=0x110) at /usr/include/c++/8.1.1/bits/stl_tree.h:759
759 (this->_M_impl._M_header._M_parent);

Trying to load the map as a game shows the error (no segfault):

Game data error
The tribe ''' does not exist.

The error does not appear when a tribe for all players are set in step 3. It doesn't matter to select a different tribe. So when just clicking the tribe button and select barbarians for all players, the segfault doesn't happen.