[In_app_purchase]fix requesthandler crash (flutter#1199)

The FIAPRequestHandler is destroyed when exiting the function. Since FIAPRequestHandler is the delegate, when SKProductRequest tries to execute the delegate method, it will not be able to find the delegate. Hence the crash.

Adding a set to hold strong reference of the FIAPRequestHandler until it is complete fix the crash.

packages/in_app_purchase/ios/Classes/InAppPurchasePlugin.m

@@ -7,6 +7,14 @@
 #import "FIAObjectTranslator.h"
 #import "FIAPRequestHandler.h"
 
+@interface InAppPurchasePlugin ()
+
+// Holding strong references to FIAPRequestHandlers. Remove the handlers from the set after
+// the request is finished.
+@property(strong, nonatomic) NSMutableSet *requestHandlers;
+
+@end
+
 @implementation InAppPurchasePlugin
 
 + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
@@ -42,6 +50,8 @@ - (void)handleProductRequestMethodCall:(FlutterMethodCall *)call result:(Flutter
   SKProductsRequest *request =
       [self getProductRequestWithIdentifiers:[NSSet setWithArray:productIdentifiers]];
   FIAPRequestHandler *handler = [[FIAPRequestHandler alloc] initWithRequest:request];
+  [self.requestHandlers addObject:handler];
+  __weak typeof(self) weakSelf = self;
   [handler startProductRequestWithCompletionHandler:^(SKProductsResponse *_Nullable response,
                                                       NSError *_Nullable error) {
     if (error) {
@@ -61,11 +71,21 @@ - (void)handleProductRequestMethodCall:(FlutterMethodCall *)call result:(Flutter
       return;
     }
     result([response toMap]);
+    [weakSelf.requestHandlers removeObject:handler];
   }];
 }
 
 - (SKProductsRequest *)getProductRequestWithIdentifiers:(NSSet *)identifiers {
   return [[SKProductsRequest alloc] initWithProductIdentifiers:identifiers];
 }
 
+#pragma mark - getter
+
+- (NSSet *)requestHandlers {
+  if (!_requestHandlers) {
+    _requestHandlers = [NSMutableSet new];
+  }
+  return _requestHandlers;
+}
+
 @end