fix(api): prevent TypeError when chart already destroyed

Add release of prototype chain methods.
Assign emtpy function to prevent any possible error throws.

Close #1613

src/Chart/api/chart.ts

@@ -101,7 +101,7 @@ export default {
 			window.removeEventListener("resize", $$.resizeFunction);
 			chart.classed("bb", false).html("");
 
-			// releasing references
+			// releasing own references
 			Object.keys(this).forEach(key => {
 				key === "internal" && Object.keys($$).forEach(k => {
 					$$[k] = null;
@@ -110,6 +110,11 @@ export default {
 				this[key] = null;
 				delete this[key];
 			});
+
+			// release prototype chains
+			for (const key in this) {
+				this[key] = () => {};
+			}
 		}
 
 		return null;

test/api/chart-spec.ts

@@ -82,7 +82,13 @@ describe("API chart", () => {
 			chart.destroy();
 
 			expect(d3Select("#chart svg").empty()).to.be.true;
-			expect(Object.keys(chart).length).to.be.equal(0);
+
+			// all methods should be ressetted
+			Object.keys(chart).forEach(key => {
+				expect(chart[key]()).to.be.undefined;
+				expect(/^function()/.test(chart[key].toString())).to.be.true;
+			});
+
 			expect(bb.instance.indexOf(chart) === -1).to.be.true;
 		});
 
@@ -103,6 +109,11 @@ describe("API chart", () => {
 				setTimeout(done, 500);
 			}, 500);
 		});
+
+		it("should not throw error when already destroyed", () => {
+			chart.destroy();
+			chart.destroy();
+		});
 	});
 
 	describe("config()", () => {