[LP-11142] Fixing a potential objc_release crash (#337)

* Fixing potential crash

* Fixing podfile, adding synchronized for startIssuedBlocks

* Adding precaution nil check

Author: milos1290

Example/Podfile

@@ -20,6 +20,6 @@ target 'Leanplum-SDK_Example' do
 
       pod 'Leanplum-iOS-SDK', :path => '../'
       pod 'OCMock', '~> 3.3.1'
-      pod 'OHHTTPStubs'
+      pod 'OHHTTPStubs', '~> 9.0.0'
   end
 end

Leanplum-SDK/Classes/Features/Events/LPEventCallback.m

@@ -57,8 +57,13 @@ - (void)invokeResponseWithOperation:(id<LPNetworkOperationProtocol>)operation
 
     // Ensure all callbacks are on main thread.
     if (![NSThread isMainThread]) {
+        // hold the reference to self until block is executed
+        __strong __typeof__(self) strongSelf = self;
         dispatch_async(dispatch_get_main_queue(), ^{
-            self.responseBlock(operation, response);
+            __typeof__(self) weakSelf = strongSelf;
+            if (weakSelf) {
+                weakSelf.responseBlock(operation, response);
+            }
         });
         return;
     }
@@ -74,8 +79,13 @@ - (void)invokeError:(NSError *)error
 
     // Ensure all callbacks are on main thread.
     if (![NSThread isMainThread]) {
+        // hold the reference to self until block is executed
+        __strong __typeof__(self) strongSelf = self;
         dispatch_async(dispatch_get_main_queue(), ^{
-            self.errorBlock(error);
+            __typeof__(self) weakSelf = strongSelf;
+            if (weakSelf) {
+                weakSelf.errorBlock(error);
+            }
         });
         return;
     }

Leanplum-SDK/Classes/Features/Events/LPEventCallbackManager.m

@@ -82,13 +82,13 @@ + (void)invokeSuccessCallbacksOnResponses:(id)responses
 
             // If index is in range, execute and remove it.
             // If not, requests are in the future. Update the index.
-            [callbackMap removeObjectForKey:indexObject];
             if (index >= requests.count) {
                 index -= requests.count;
                 updatedCallbackMap[@(index)] = eventCallback;
             } else if (eventCallback.responseBlock) {
                 activeResponseMap[indexObject] = [eventCallback.responseBlock copy];
             }
+            [callbackMap removeObjectForKey:indexObject];
         }
         [callbackMap addEntriesFromDictionary:updatedCallbackMap];
 

Leanplum-SDK/Classes/Internal/Leanplum.m

@@ -410,10 +410,12 @@ + (void)startWithUserId:(NSString *)userId userAttributes:(NSDictionary *)attrib
 + (void)triggerStartIssued
 {
     [LPInternalState sharedState].issuedStart = YES;
-    for (LeanplumStartIssuedBlock block in [LPInternalState sharedState].startIssuedBlocks.copy) {
-        block();
+    @synchronized ([LPInternalState sharedState].startIssuedBlocks) {
+        for (LeanplumStartIssuedBlock block in [LPInternalState sharedState].startIssuedBlocks.copy) {
+            block();
+        }
+        [[LPInternalState sharedState].startIssuedBlocks removeAllObjects];
     }
-    [[LPInternalState sharedState].startIssuedBlocks removeAllObjects];
 }
 
 + (void)triggerStartResponse:(BOOL)success
@@ -1274,10 +1276,12 @@ + (void)onStartIssued:(LeanplumStartIssuedBlock)block
     if ([LPInternalState sharedState].issuedStart) {
         block();
     } else {
-        if (![LPInternalState sharedState].startIssuedBlocks) {
-            [LPInternalState sharedState].startIssuedBlocks = [NSMutableArray array];
+        @synchronized ([LPInternalState sharedState].startIssuedBlocks) {
+            if (![LPInternalState sharedState].startIssuedBlocks) {
+                [LPInternalState sharedState].startIssuedBlocks = [NSMutableArray array];
+            }
+            [[LPInternalState sharedState].startIssuedBlocks addObject:[block copy]];
         }
-        [[LPInternalState sharedState].startIssuedBlocks addObject:[block copy]];
     }
 }