Fixing null check error

[tashigarg]

Fixing null check error Seeing error from library when moving the splitter

ReflexContainer.js?95b3:89 Uncaught TypeError: Cannot read property 'map' of null at ReflexEvents.eval (ReflexContainer.js?95b3:89) at ReflexEvents.emit (ReflexEvents.js?3497:63) at HTMLDocument.eval (ReflexSplitter.js?a489:92)

Code Usage `

          <ReflexSplitter propagate={true} onStopResize={args => {
            this.setState({
              dockSize: this.getSize()
            })
          } }/>
          <ReflexElement size={this.state.dockSize} direction={-1} minSize={DOCK_MIN_SIZE}>
           <DockView onMaximizeClicked={this.onMaximizeClicked} onMinimizeClicked={this.onMinimizeClicked}/>
          </ReflexElement>
        </ReflexContainer>

`