Fix crash when React Native is reloaded

Though the crash is fixed and the node is properly stopped,
things don't work properly afterwards.
For instance estimating fee fails with the following error:
"Tx gas price does not exceed minimum"

ios/ReactNativeGeth.swift

@@ -26,6 +26,15 @@ class ReactNativeGeth: RCTEventEmitter, GethNewHeadHandlerProtocol {
         self.geth_node = NodeRunner()
         super.init()
     }
+
+    // Called when React Native is reloaded
+    @objc func invalidate() {
+        do {
+            try geth_node.getNode()?.stop()
+        } catch {
+            NSLog("Failed stopping geth node: \(error)")
+        }
+    }
 
     @objc(supportedEvents)
     override func supportedEvents() -> [String]! {
@@ -44,6 +53,12 @@ class ReactNativeGeth: RCTEventEmitter, GethNewHeadHandlerProtocol {
     }
 
     func onNewHead(_ header: GethHeader) {
+        guard bridge != nil else {
+            // Don't call sendEvent when the bridge is not set
+            // this happens when RN is reloaded and this module is unregistered
+            return
+        }
+
         do {
             let json = try header.encodeJSON()
             let dict = try self.convertToDictionary(from: json)