ensure first frame key is always valid

Auditors: @NejcZdovc
fix #11208
Test Plan:
npm run test -- --grep="isFirstFrameKeyIntabPage"

js/state/frameStateUtil.js

@@ -480,7 +480,7 @@ const isFirstFrameKeyInTabPage = (state, frameKey) => {
   const firstFrame = unpinnedTabs
     .slice(startingFrameIndex, startingFrameIndex + tabsPerTabPage).first()
 
-  return firstFrame.get('key') === frameKey
+  return !!firstFrame && firstFrame.get('key') === frameKey
 }
 
 const getTabPageIndex = (state) => {

test/unit/state/frameStateUtilTest.js

@@ -444,4 +444,40 @@ describe('frameStateUtil', function () {
       assert.equal(result, true)
     })
   })
+
+  describe('isFirstFrameKeyInTabPage', function () {
+    beforeEach(function () {
+      this.frameKey1 = 1
+      this.frameKey2 = 7
+      this.state = defaultWindowStore.mergeIn(['frames'],
+        [{key: this.frameKey1}, {}, {}, {}, {}, {}, {key: this.frameKey2}]
+      )
+    })
+    it('returns false if frames are undefined', function () {
+      getSettingsValue = 10
+      const nullFrames = this.state.mergeIn(['frames'], [{}])
+      const result = frameStateUtil.isFirstFrameKeyInTabPage(this.state, nullFrames)
+      assert.equal(result, false)
+    })
+    it('returns true if the frame is the first in the first tab set', function () {
+      getSettingsValue = 10
+      this.state = this.state.setIn(['ui', 'tabs', 'tabPageIndex'], 0)
+      const result = frameStateUtil.isFirstFrameKeyInTabPage(this.state, this.frameKey1)
+      assert.equal(result, true)
+    })
+    it('ignores pinned frames even if frame is the first in the tab set', function () {
+      getSettingsValue = 10
+      this.state = this.state
+        .setIn(['ui', 'tabs', 'tabPageIndex'], 0)
+        .mergeIn(['frames', 0], {pinnedLocation: true})
+      const result = frameStateUtil.isFirstFrameKeyInTabPage(this.state, this.frameKey1)
+      assert.equal(result, false)
+    })
+    it('returns true if the frame is the first in the second tab set', function () {
+      this.state = this.state.setIn(['ui', 'tabs', 'tabPageIndex'], 1)
+      getSettingsValue = 6
+      const result = frameStateUtil.isFirstFrameKeyInTabPage(this.state, this.frameKey2)
+      assert.equal(result, true)
+    })
+  })
 })