Fixes Layer.isPopupOpen when no popup is bound (#5106)

* Adds initial tests for L.Layer#popup

* Fixes Layer.isPopupOpen when no popup is bound

This fixes uncaught exception when checking whether a popup isOpen(),
when the layer has no popup bound to it.

spec/suites/layer/PopupSpec.js

@@ -328,3 +328,67 @@ describe("L.Map#openPopup", function () {
 	});
 
 });
+
+describe('L.Layer#_popup', function () {
+	var c, map, marker;
+
+	beforeEach(function () {
+		c = document.createElement('div');
+		c.style.width = '400px';
+		c.style.height = '400px';
+		map = new L.Map(c);
+		map.setView(new L.LatLng(55.8, 37.6), 6);
+		marker = L.marker(L.latLng(55.8, 37.6)).addTo(map);
+	});
+
+	afterEach(function () {
+		if (document.body.contains(c)) {
+			document.body.removeChild(c);
+		}
+	});
+
+	it("only adds a popup to the map when opened", function () {
+		marker.bindPopup("new layer");
+		expect(map.hasLayer(marker.getPopup())).to.be(false);
+		marker.openPopup();
+		expect(map.hasLayer(marker.getPopup())).to.be(true);
+	});
+
+	it("keeps an open popup on the map when it's unbound from the layer", function () {
+		marker.bindPopup("new layer").openPopup();
+		var popup = marker.getPopup();
+		marker.unbindPopup();
+		expect(map.hasLayer(popup)).to.be(true);
+	});
+
+	it("should not give an error when the marker has no popup", function () {
+		expect(function () {
+			marker.isPopupOpen();
+		}).to.not.throwException();
+		expect(marker.isPopupOpen()).to.be(false);
+	});
+
+	it("should show a popup as closed if it's never opened", function () {
+		marker.bindPopup("new layer");
+		expect(marker.isPopupOpen()).to.be(false);
+	});
+
+	it("should show a popup as opend if it's opened", function () {
+		marker.bindPopup("new layer").openPopup();
+		expect(marker.isPopupOpen()).to.be(true);
+	});
+
+	it("should show a popup as closed if it's opened and closed", function () {
+		marker.bindPopup("new layer").openPopup().closePopup();
+		expect(marker.isPopupOpen()).to.be(false);
+	});
+
+	it("should show the popup as closed if it's unbound", function () {
+		marker.bindPopup("new layer").openPopup().unbindPopup();
+		expect(function () {
+			marker.isPopupOpen();
+		}).to.not.throwException();
+		expect(marker.isPopupOpen()).to.be(false);
+	});
+
+});

src/layer/Popup.js

@@ -453,7 +453,7 @@ L.Layer.include({
 	// @method isPopupOpen(): boolean
 	// Returns `true` if the popup bound to this layer is currently open.
 	isPopupOpen: function () {
-		return this._popup.isOpen();
+		return (this._popup ? this._popup.isOpen() : false);
 	},
 
 	// @method setPopupContent(content: String|HTMLElement|Popup): this