Add check for currentProps before calling

[ZheSun88]

This is aiming to fix the client side error for vaadin/vaadin-upload-flow#51.

The upload component has a dom-repeat for its items internally. The
upload creates a change on the internal list, but with a JS component
that doesn't have a nodeID. On flow side, we don't check whether it has
a nodeID or not, but asking, which will throw an error.

After we remove the item from the component, the item is removed from
the currentProps, which becomes undefined, on flow side, we should have
checked that first, otherwise the error will be thrown again.

Test for the component will be added after this PR.

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This change is 

[denis-anisimov]

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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flow-client/src/main/java/com/vaadin/client/flow/binding/SimpleElementBindingStrategy.java, line 351 at r1 (raw file):

currentPropsItem != undefined

This is not a really check for undefined but you don't need this check.
You need a check whether currentPropsItem has a value which should be done just as
if ( currentPropsItem && currentPropsItem.nodeId )

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Comments from Reviewable

[ZheSun88]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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flow-client/src/main/java/com/vaadin/client/flow/binding/SimpleElementBindingStrategy.java, line 351 at r1 (raw file):

Previously, denis-anisimov (Denis) wrote…

currentPropsItem != undefined

This is not a really check for undefined but you don't need this check.
You need a check whether currentPropsItem has a value which should be done just as
if ( currentPropsItem && currentPropsItem.nodeId )

got it. Thanks.

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Comments from Reviewable

[vaadin-bot]

SonarQube analysis reported 5 issues

Note: The following issues were found on lines that were not modified in the pull request. Because these issues can't be reported as line comments, they are summarized here:

1. SimpleElementBindingStrategy.java#L194: Make the enclosing method "static" or remove this set.
2. SimpleElementBindingStrategy.java#L993: Remove this unused private "invokeWhenNodeIsConstructed" method.
3. SimpleElementBindingStrategy.java#L1300: Equality tests should not be made with floating point values.
4. SimpleElementBindingStrategy.java#L194: Write to static field com.vaadin.client.flow.binding.SimpleElementBindingStrategy.boundNodes from instance method com.vaadin.client.flow.binding.SimpleElementBindingStrategy.bind(StateNode, Element, BinderContext)
5. SimpleElementBindingStrategy.java#L415: Complete the task associated to this TODO comment.

[denis-anisimov]

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Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable