Compile error in arduino IDE of the library

[pvdw0310]

I made years ago some programs for the esp8266 with a nextion touch screen. It is still working and I like to change some things in that program. But now I get some compiler errors and I don't find the problem.

In nexRtc.cpp I got 3 errors that there was no return in some functions.
-> I change the file with some extra returns like :

 cmd += sec;
 sendCommand(cmd.c_str());
 recvRetCommandFinished();

-> return (true); // I added this otherwise compile error

Then I get the next error:
C:\Users\peter\Documents\Arduino\libraries\ITEADLIB_Arduino_Nextion\NexRtc.cpp: In member function 'bool NexRtc::write_rtc_time(char*)':
C:\Users\peter\Documents\Arduino\libraries\ITEADLIB_Arduino_Nextion\NexRtc.cpp:20:30: error: control reaches end of non-void function [-Werror=return-type]
20 | String cmd = String("rtc");

But now I don't know what I can do to get around the problem?
Is there somebody that can point me in the direction of some solution?
I have also a similar error in nexUpload.cpp.
Thanks.