crash any other threads with an error message

crash any other threads with an error message if signals are acessed from that thread. 

It would be better to have a solution that makes it so that at compile time users would get an error if signals are used improperly, but at least now there will be an error message

reactive/src/runtime.rs

@@ -3,6 +3,7 @@ use std::{
     cell::{Cell, RefCell},
     collections::{HashMap, HashSet},
     rc::Rc,
+    sync::atomic::{AtomicBool, Ordering},
 };
 
 use smallvec::SmallVec;
@@ -13,8 +14,14 @@ use crate::{
     signal::Signal,
 };
 
+static CREATED: AtomicBool = AtomicBool::new(false);
 thread_local! {
-    pub(crate) static RUNTIME: Runtime = Runtime::new();
+    pub(crate) static RUNTIME: Runtime = {
+        if CREATED.compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst).is_err() {
+            panic!("RUNTIME must only be created once. You are probably using signals from multiple threads. All signals need to be accessed exclusively from the main thread.");
+        }
+        Runtime::new()
+    };
 }
 
 /// The internal reactive Runtime which stores all the reactive system states in a