Ensure only one cell shows as running at a time (#4715)

src/sql/parts/notebook/models/cell.ts

@@ -239,6 +239,7 @@ export class CellModel implements ICellModel {
 			this.sendNotification(notificationService, Severity.Error, message);
 			// TODO track error state for the cell
 		} finally {
+			this.disposeFuture();
 			this.fireExecutionStateChanged();
 		}
 
@@ -324,8 +325,7 @@ export class CellModel implements ICellModel {
 		let output: nb.ICellOutput = msg.content as nb.ICellOutput;
 
 		if (!this._future.inProgress) {
-			this._future.dispose();
-			this._future = undefined;
+			this.disposeFuture();
 		}
 	}
 
@@ -487,4 +487,10 @@ export class CellModel implements ICellModel {
 		}
 		return endpoint;
 	}
+
+	// Dispose and set current future to undefined
+	private disposeFuture() {
+		this._future.dispose();
+		this._future = undefined;
+	}
 }