fix: prevent dedup crash

CHANGELOG.md

@@ -1,5 +1,15 @@
 **Note:** This is a cumulative changelog that outlines all of the Apollo Link project child package changes that were bundled into a release on a specific day.
 
+## 2019-03-14
+
+### apollo-link-dedup 1.0.18
+
+- Fixes an issue introduced in [#984](https://github.com/apollographql/apollo-link/pull/984)
+  where subscriber `next` and/or `error` calls might have already deleted the 
+  key the new dedupe changes were intended to help with.  <br/>
+  [@JoviDeCroock](https://github.com/JoviDeCroock) in [#988](https://github.com/apollographql/apollo-link/pull/988)
+
+
 ## 2019-03-13
 
 ### apollo-link-dedup 1.0.17

packages/apollo-link-dedup/src/dedupLink.ts

@@ -63,10 +63,12 @@ export class DedupLink extends ApolloLink {
         }
 
         return () => {
-          this.subscribers.get(key).delete(observer);
-          if (this.subscribers.get(key).size === 0) {
-            this.inFlightRequestObservables.delete(key);
-            if (subscription) subscription.unsubscribe();
+          if (this.subscribers.has(key)) {
+            this.subscribers.get(key).delete(observer);
+            if (this.subscribers.get(key).size === 0) {
+              this.inFlightRequestObservables.delete(key);
+              if (subscription) subscription.unsubscribe();
+            }
           }
         };
       });