fix: Fix view-not-found race condition in C++ code (#511)

* Add custom `onViewReady` event to get layout

`componentDidMount` is async, so the native view _might_ not exist yet causing a race condition in the `setFrameProcessor` code.

This PR fixes this by calling `setFrameProcessor` only after the native view has actually mounted, and to ensure that I created a custom event that fires at that point.

* Update CameraView.swift

ios/CameraView.swift

@@ -65,6 +65,7 @@ public final class CameraView: UIView {
   @objc var onInitialized: RCTDirectEventBlock?
   @objc var onError: RCTDirectEventBlock?
   @objc var onFrameProcessorPerformanceSuggestionAvailable: RCTDirectEventBlock?
+  @objc var onViewReady: RCTDirectEventBlock?
   // zoom
   @objc var enableZoomGesture = false {
     didSet {
@@ -77,7 +78,7 @@ public final class CameraView: UIView {
   }
 
   // pragma MARK: Internal Properties
-
+  internal var isMounted = false
   internal var isReady = false
   // Capture Session
   internal let captureSession = AVCaptureSession()
@@ -179,6 +180,17 @@ public final class CameraView: UIView {
                                               object: nil)
   }
 
+  override public func willMove(toSuperview newSuperview: UIView?) {
+    super.willMove(toSuperview: newSuperview)
+    if !isMounted {
+      isMounted = true
+      guard let onViewReady = onViewReady else {
+        return
+      }
+      onViewReady(nil)
+    }
+  }
+
   // pragma MARK: Props updating
   override public final func didSetProps(_ changedProps: [String]!) {
     ReactLogger.log(level: .info, message: "Updating \(changedProps.count) prop(s)...")