it's not thread safe

to reproduce
```
for(int i=1;i<100;i++)
std::thread(test1).detach();
```
```
void test1()
{
Timer t = Timer();
t.setInterval([&](){
},
1000);
t.stop();
}
```
results
Exception has occurred.
Segmentation fault

Start by reproducing the reported segmentation fault with the provided C++ snippet, then inspect the Timer implementation and its handling of concurrent construction, setInterval, and stop. Done means the repeated multithreaded case no longer segfaults.

Written by the indexing model from the issue text.